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Washington, I). C., October 1, 1885. 

SIK : I have the honor to transmit herewith the manuscript of a report 
on the Geology and Mining Industry of Leadville, Colorado. 

To yourself, and to the Hon. Clarence King, under whose direction 
this investigation was commenced, I am greatly indebted for the facilities 
and kind encouragement that have always been afforded to those engaged 
in its prosecution. 

Very respectfully, your obedient servant, 


Geologist-in- Charge. 
Hon. J. W. POWELL, 

Director United Slates Geological -Sarvey, Washington, D. C. 


The present work was undertaken at the instance of the Hon. Clarence 
King, first Director of the United States Geological Survey, in 1879. It 
was his intention that it should form part of a series of monographs which 
would in time include all the important mining districts of the country, and 
thus furnish an accurate and permanent record of the manner of occurrence 
and geological relations of the metallic deposits of the United States, as well 
as of all substantial improvements in the methods of obtaining the metals 
from their ores. 

In preparing such a monograph the general plan adopted was : first, to 
obtain an accurate knowledge of the geological structure of the region and 
of the various rocks of which it is made up ; next, to study thoroughly the 
ore deposits in their varied relations to the inclosing rocks ; and, finally, to 
investigate any methods of extraction or of reduction of the ores that pre- 
sented new or unusual features, without wasting time upon what was already 
so well known as to require no further comment. Various circumstances 
rendered such modifications of this plan necessary in the present case that 
the various stages of the work could not always be carried on in their log- 
ical sequence. The great altitude of the region and consequent inclemency 
of its climate practically prevented surface work being carried on to ad- 
vantage during eight months of the year. The organization of the Survey 
was as yet incomplete, and assistants familiar with this class of work could 
not immediately be obtained ; moreover, a year elapsed after the inception 
of the work before laboratory facilities could be obtained which rendered 



it possible to carry on the chemical investigations that form one of its most 
important and essential features. The first want was accurate and detailed 
topographical maps, which are more than usually indispensable in the 
vicinity of Leadville, where the entire rock surface is covered by ddbris, and 
the geological structure had to be reconstructed by gathering into a con- 
nected whole the data derived from thousands of isolated shafts and tunnels 
which had penetrated below the surface accumulations. 

This want was supplied by Chief Topographer A. D. Wilson, the une- 
qualed accuracy and rapidity of whose work can only be adequately appre- 
ciated by those who have had occasion, as we had, to put it to the test of 
actual instrumental verification. The field work of the map of Leadville and 
vicinity was completed by him and his two assistants during the months of 
August and September, 1879, and that of the map of Mosquito Range during 
part of July, August, and September, 1880. 

In December, 1879, 1 commenced the study of the ore deposits of Lead- 
ville. In this I received most invaluable aid from Mr. Ernest Jacob, grad- ' 
uate of the Royal School of Mines of London, who, working at first as 
volunteer, rendered most continuous and unwearied service during the whole 
continuance of the investigation To his keen insight into the intricacies 
of geological structure, his untiring energy in exploring every accessible 
prospect-hole in the region, and his accurate appreciation of the bearing 
of the data thus gathered, is attributable in great measure the successful 
unraveling of the complicated problem presented in the region represented 
on the map of Leadville and vicinity. So complicated a region, I make 
bold to say, it rarely falls to the lot of a geologist to study in detail. 

In July, 1880, it was first practicable to undertake the study of the 
high mountain region represented on the map of the Mosquito Range. 
Here geological and topographical field work went hand in hand, and my 
party worked together with that of Mr. Wilson until heavy snows at the 
end of September put an end to outside work. In this field work I had 
the assistance of Mr. Whitman Cross, who had made a special study of 
microscopical petrography under Professor Zirkel, of Leipzig, and of Prof. 
Arthur Lakes, of the School of Mines at Golden, Colo., who devoted his 
summer vacation to this work. To Mr. Cross, who, like Mr. Jacob, first 


joined the Survey as volunteer assistant, was intrusted the final petro- 
graphical determination of all the crystalline rocks of the region, and the 
great value of his subsequent investigations in the field of petrography and 
mineralogy have fully justified the confidence thus placed in his ability. 

In the autumn of 1880 the corps was increased by the addition of Mr. 
W. F. Hillebrand, who had already distinguished himself by his original 
investigations in inorganic chemistry in the laboratory of Professor Bunsen 
at Heidelberg; under his direction a laboratory was prepared at Denver in 
connection with the headquarter offices of this division of the Survey. 

During the summer I was fortunate enough to secure the services of 
Mr. Antony Guyard, a former pupil of the Ecole des Mines, and for twelve 
years chemist at the well known metallurgical works of Johnson & Mattey, 
London. At my request Mr. Guyard undertook the labor of making a 
chemical investigation of the processes of lead smelting as conducted at 
the various Leadville smelters. His sudden death at Paris, which was 
closely followed by that of his brother Stanislas, the distinguished French 
Orientalist, prevented the personal revision of his report which I could have 
desired him to make; and in that which was made by Mr. Hillebrand and 
myself we have not always felt justified in making modifications which 
might have been judged advisable could we have discussed the points with 
the author himself. Beyond the correction of a few clerical errors it is pre- 
sented substantially in the form in which it was left by him. 

In November, 1880, Messrs. Hillebrand and Guyard commenced their 
respective chemical investigations, the one of the rocks and ores, the other 
of the furnace products of Leadville, in the laboratory at Denver. 

Mr. W. H. Leffingwell, with the assistance of Mr. Jacob, completed the 
Leadville map during the latter part of 1880 by the accurate location of 
various shafts and tunnels, to the number of nearly a thousand, found 
necessary for the determination of the geological outlines, an extremely 
laborious undertaking, carried on as it was at times with 15 to 20 feet of 
snow on the ground. 

About the same time the topography and underground workings of the 
maps of Iron, Carbonate, and Fryer Hills were prepared under my direc- 


tion by Messrs. H. Huber & Co., F. G. Bulkley & Co., and George H. 
Robinson & Co., respectively. 

From June, 1880, to June, 1881, my time was partially taken up in 
tb^ supervision and direction of experts employed under the authority of 
the Superintendent of the Census in making an investigation into the "Sta- 
tistics and Technology of the Precious Metals " in the Rocky Mountains. 

From the close of field work in the summer of 1880 to May, 1881, I 
was mainly occupied with Mr. Jacob in completing the examination of the 
mines and deposits of Leadville In this work we received, with a single 
exception, the most courteous treatment from mine owners and superin- 
tendents, who not only opened their mines freely to our inspection and per- 
mitted the use of the maps of their underground workings, but also aided 
us materially in many cases by the information they furnished from their 
own every-day experience To these gentlemen, individually and collect- 
ively, I return my most hearty thanks, as well for the services above 
mentioned as for the confidence thereby displayed in the disinterestedness 
of our motives and our wish to be of service to the mining public in gen- 
eral without favoring unduly any individual or corporation. 

During the summer of 1881 the individual members of the corps, aided 
by Messrs. Morris Bien and W. B v. Richthofen, were occupied in collating 
the results obtained, and in the preparation of the various maps and illus- 
trations for the engraver, and by autumn the work was so far completed 
that I was enabled to embody the principal results arrived at in an abstract 
published in the Second Annual Report of the Director of the Survey. 

During the time that has elapsed since the publication of that abstract 
the development of the Leadville mines has proceeded with rapid strides, 
and already the ores are changing from carbonates and chlorides to sul- 
phides. In other respects also these developments have afforded most 
gratifying confirmation of the general accuracy of the geological outlines 
given on the accompanying maps and sections. Even had it been other- 
wise, it would have been impracticable to have changed what had long since 
been engraved. In the press of other work it was not possible to attempt 
another examination of the field, and therefore in the final revision of this 


long-delayed material the changes have been mainly confined to condens- 
ing and leaving out what has in a measure lost its value by the lapse of 
time. Where new facts have been obtained, they have been inserted in 
notes. The report as it now stands is therefore essentially that which was 
prepared four years ago, and as such it should be criticised by those who have 
occasion to read it. 

WASHINGTON, October 1, 1885. 












Topographical description 3 

Routes of approach g 

Discovery of the precious metals 7 

Development of mines f 10 

Growth of the city 14 

Production 15 



Rocky Mountains in Colorado 19 

Eastern uplift 20 

The Parks 22 

Western uplift 23 

Mountain structure 24 

Mosquito Range Topography 27 

Geological history 30 

Mineral deposition 33 

Structural results of the dynamic movements 34 

Displacement Volcanic rocks 39 

General erosion Arkansas Valley erosion 40 

Glacial erosion 41 

Stream erosion Valleys 42 






Sedimentary rocks 45 

Archeau format ious 45 

Granite >. 4g 

Gneiss 48 

Amphibolite 50 

Relative age 51 

Paleozoic formations 53 

('aml>ri:tu 58 

Lower Quartzite 58 

iSiluriiiii tjO 

White Limestone 60 

Parting Quartzite (il 

Corresponding beds in Colorado Range 62 

Carboniferous 63 

Blue or ore-beating limestone 63 

Its composition 04 

Weber Shali-s 67 

Weber Grits 68 

Upper Coal Measures . 69 

Mesozoic formations 70 

Quaternary formations 71 

Glacial or Lake beds 71 

Recent or post-glacial 72 

Distribution of sedimentary formations 72 

Eruptive or igneous rocks 74 

Secondary cruptives 74 

Mount Zion Porphyry White Porphyry 7(i 

Lincolu Porphyry 78 

Ci-.-iy Porphyry 80 

Sacramento Porphyry 81 

Pyritiferous Porphyry 82 

Mosquito Porphyry Green Porphyry Silverheels Porphyry 83 

Diorite 84 

Porphyrite 85 

Tertiary cruptives 86 

Rhyolite 87 

Trachyte Andesite 83 



Introductory 90 

Surface features 91 

Glacial formations 92 

Post-Glacial formations Archeau exposures !3 

Northeastern division 94* 

Platte amphitheater 94 

Quandary Peak 96 

Hoosier pass ridge 100 

Silverheels Massive 104 

Lincoln Massive 107 

Cameron amphitheater Ill 


Descriptive geology of the Mosquito Range Continued. 

Northeastern division Continued. 

Mounts Cameron and Bross 115 

Red amphitheater 119 

Eastern foot-hills 122 

Buckskin amphitheater 123 

North Mosquito amphitheater 125 

Middle-eastern division 126 

Glacial erosiou 126 

Buckskin section 128 

Loveland Hill 130 

North Mosquito section 131 

South Mosquito section 133 

Pennsylvania Hill 135 

London fault 136 

Main crest from Mosquito Peak to Mount Evans 139 

South Mosquito amphitheater Sacramento amphitheater 140 

London Hill 141 

Pennsylvania Hill west of London fault 144 

Sacramento arch 147 

Gemini Peaks 149 

Little Sacramento gulch 151 

Spring Valley Horseshoe gulch 152 

White Ridge ". 153 

North wall of Horseshoe gulch 155 

Four-Mile amphitheater 158 

The Horseshoe 160 

South wall of Horseshoe gulch 161 

Lamb Mountain 163 

Sheep Mountain ^ 165 

Southern division 169 

Sheep Ridge 169 

Black Hill 170 

Twelve-Mile Creek 171 

Weston's pass 173 

South Peak Ridge 174 

Western slopes 175 

Weston fault Empire Hill 176 

Empire gulch 181 

Main crest north of Ptarmigan Peak 182 

Lake beds 183 

Northwestern division 184 

Prospect Mountain 184 

Mount Zion Porphyry 185 

Mount Zion -. 186 

Tennessee Park 189 

East Arkansas Valley , 190 

Buckeye Peak 193 

Chalk Mountain 194 

Upper Ten-Mile Valley 197 

Mosquito fault 198 

Arkansas amphitheater 199 

Ten-Mile and Clinton amphitheaters 201 





Geiieral struct nre 202 

Distribution of porphyry bodies 206 

White Porphyry 206 

Gray Porphyry 207 

Pyritiferous Porphyry Other porphyries 208 

Area east of Mowjnito fault 209 

Frontispiece 210 

Mosquito fault Minor faults 211 

West Sheridan Dyer Mountain 212 

Evans amphitheater 214 

East Ball Mounain 215 

Area between Mosquito and Ball Mountain faults 215 

Ball Mountain fault Prospect Mountain Ridge 215 

Little Ellen Hill Eruptive dikes 216 

Coal in Weber Shales Blue Limestone 217 

South slope of Ball Mountain 219 

Area between Ball Mountain and West on faults 219 

Weston fault 220 

Iowa fault Southwest slope of Ball Mountain 221 

Northwest slope of Ball Mountain 223 

Colorado Prince fault 224 

South Evans anticline 225 

Area between Weston and Mike faults 226 

Mike fault 226 

Pilot fault 227 

Union fault Long and Derry Ridge 228 

Long and Derry mines Dikes Iowa gulch 230 

Printer Boy Hill 231 

Head of California gulch Pyritiferons Porphyry 233 

North side of California gulch Printer Boy Porphyry 234 

Mike mine Breece Hill 235 

Breece fault 236 

Breece Iron mine 287 

Area north of Breece fault 237 

Syncline east of Yankee Hill 238 

Yankee Hill anticliue North slope of Yankee Hill 240 

South slope of Yankee hill 241 

Adelaide fault Southwest slope of Yankee Hill 243 

Moraines ^44 

Area between Mike and Iron-Dome faults 244 

Iron Dome fault 244 

Long and Derry Ridge Josephine Porphyry 

Lake beds Iowa gulch Dome Ridge 246 

South slope of Iron Hill 247 

North Iron Hill 248 

Area between Iron-Dome and Carbonate faults 248 

Carbonate fault 248 

South of California gulch Proof of synclinal fold 249 

Emmet fault Graham Park 250 

California gulch 252 

Carbonate Hill 253 

Little Stray Horse syncline 

Eastern rim 2o4 

Center of basin Western rim , 255 


Descriptive geology of Leadville and vicinity Continued. 

Fryer Hill 255 

Prospect Mountain 257 

Crest of the ridge 257 

Southern slope 258 

Little Evans anticline 259 

Yankee Hill anticline Little Stray Horse syncline Big Evans anticline 260 

Area west of Carbonate and Fryer Hills 261 

General structure 261 

Eastern rim of basin 262 

Western rim 263 

Explanation of transverse sections 263 



Sedimentary rocks 276 

Archean 276 

Paleozoic 277 

Dolomitic sediments 278 

Serpentine 281 

Origin of the serpentine 282 

Structural features 284 

Folds and faults 284 

Hade of faults > 287 

The one-sided or S-shaped fold 290 

Eruptive rocks , 292 

Age 293 

Manner of occurrence Intrusive sheets 295 

Dikes 296 

Relation of form to composition 297 

Amount of intrusive force 298 

Source of intrusive force 299 

Why intrusive and not surface flows T 1500 

Internal structure 302 

Orthoelastic and plagioclastic rocks 304 

Distribution of intrusive rocks in the Rocky Mountains 305 

Contact metaraorphism 307 

Non-absorption of sedimentary rocks by eruptive masses 308 



Discussion of classification in general 319 

Classification of Mosquito Range eruptives 322 


Quartz-Porphyry 323 

Mount Zion Porphyry 323 

White or Lead ville Porphyry 324 

Pyritiferous Porphyry . 326 

Mosquito Porphyry 327 

Lincoln Porphyry 328 

Gray Porphyry 330 



Older eruptives Continued. 

Diorite ................................................................................... 333 

Quartz-mica-diorite Hornbleude-diorite ............................................. 333 

Augite-bearing diorite ................................................................ 334 

Porphyrite ............................................................................... 334 

Principal group ............... . ...................................................... 335 

Sacramento Porphyrite ............................................................... 341 

Silverheels Porphyrite ................................................................ 342 

Miscellaneous porphyrites ............................................................. 343 

YOUNGER ERUPTIVES ......................................................................... 345 

Rhyolite ................................................................................. 345 

Chalk Mountain Nevadite ............................................................ 345 

Black Hill Rhyolite .................................................................. 349 

McXulty gulch Rhyolite ............................. , ................................ 350 

Empire gulch Rhyolite ....................................................... ........ 351 

Other rhyolites ....................................................................... 352 

Rhyolitic tufa Dike in Ten-Mile amphitheater Breccia .......................... 352 

Qnartziferous trachyte ................................................................ 352 

Audesite ................................................................................. 353 

Pyroxene-bearing hornblende-audesite ................................................. 353 

Hypprsthene-andesite Tufaceons andesites ........................................... 354 

RESUME ..................................................................................... 354 

Rock structures observed Individual rock types .......................................... 355 

Mutual relations of rock types Rock constituents Their decomposition .................. 356 

Negative observations Chemical composition ---- ........................................ 357 

NOTES UPON TUB HENRY MOUNTAIN ROCKS ................................................... 35U 

Hornblendic rocks ..................................... .................................. 359 

Augitic rocks ............................................................................ 361 

Re'snme' .................................................................................. 362 



ORE DEPOSITS ................................................................................ 367 

Classification of ore deposits in general ................................................... 307 

Leadville deposits ............................................. - ................ ......... 375 

Manner of occurrence ................................................................ 375 

Composition ......................................................................... 376 

Distribution ......................................................................... 377 

Secondary alteration Mode of formation ............................................. 378 

Age of deposits Origin of the metallic contents ...................................... 379 


IRON HILL GROUP OP MINES ................................................................. 360 

Iron Hill ................................................................................. 380 

General description ......................................................... - ........ 380 

Geological structure .. ............................................................ 3^1 

Later intrusive sheets ............................................................ 382 

White Porphyry .................................................................. ''-'' 

Blue Limestone Silurian Cambrian Iron fault ................................ ; '~l 

California fault ................................................................. 385 

Dome fault Emmet fault Dome Hill ........................................... 386 

Ore deposits ...................................................................... 388 


Irou Hill group of mines Continued. 

Iron Hill Continued. 

Mine workings 389 

Rock aiid Dome 389 

La Plata, Stone, and A. Y 390 

Lime and Smuggler 391 

South Bull's Eye and Silver Cord Combination 392 

Iron mine proper 394 

Relation of Irou fault to ore bodies 399 

North Iron Hill 401 

General geological structure 401 

Iron fault Adelaide fault Rock formations 402 

Ore deposits 404 

Mine workings , 405 

Argentine Argentine tunnel 405 

Adelaide 406 

Double Decker 408 



General structure 409 

Rock formations 409 

Carbonate fault 410 

Pendery fault Morning Star fault Ore deposits 411 

Southwest slope of Carbonate Hill 412 

Southern group of mines 414 

Carbonate workings 416 

Carbonate incline 418 

Little Giant 424 

Yankee Doodle 425 

Area ou top of hill Area west of Carbonate fault , 427 

Northern group of mines 429 

Crescent mine 430 

Catalpa mine 431 

Evening Star mine 433 

Morning Star mine 436 

Area west of Carbonate fanlt 439 

Lower Henriett and Waterloo 440 

Morning Star and Forsaken 441 

Niles-Augusta and Wild Cat 443 

Lower Crescent 444 



General description 445 

Rock formations 447 

Gray Porphyry 447 

White Porphyry 448 

Weber quartzite Blue Limestone 44t> 

Gaugne Ore deposits 451 

Parting Qnartzite White Limestone Lower Quartzite 453 

Explanation of Fryer Hill map 454 

Mine workings 4.V> 

Chrysolite mine 455 


Fryer Hill group of mines Continued. 

Mine workings Continued. 

New Discovery mine .................................................................. 462 

Little Chief mine .................................................................... 465 

Gray Porphyry dike .............................................................. 466 

Little Pittsburgh mine ................................................ ................ 468 

Amir mine ........................................................................... 471 

Climax mine ............ . ........................................................... 475 

Virginins mine ....................................................................... 477 

Dnnkinmine ......................................................................... 478 

Matchless mine ....................................................................... 480 

Hiberniaand Big Pittsburgh .......................................................... 482 

Robert E. Lee mine ................ . ........ v ......................................... 483 

Little Sliver Southeast corner of region mapped ..................................... 485 

Little Stray Horse gulch .............. , ............................................... 488 

Re'sume' .................................................................................. 489 


OTHER GROUPS OF MINES ..................................................................... 493 

Mines and prospects in the Leadvillo region ................................................ 493 

Little Stray Horse synoline ........................................................... 495 

Southern rim ..................................................................... 493 

Southeastern rim Western rim ................................................... 496 

Eastern rim ...................................................................... 497 

Yankee Hill anticline ................................................................. 496 

Breece Iron mine ................................................................. 499 

Syncline east of Yankee Hill ......................................................... 500 

South Evans anticline ................................................................ 501 

Highland Chief mine ............................................................. 501 

Colorado Prince group ............................................................ 503 

Little Ellon Hill ................. .. .................................................. 506 

Breece Hill ........................................................................... 506 

Green Mountain .................... . .................... . ............................ 507 

Long and Deny Hill Ready Cash mine Long and Derry mines ...................... 50rf 

Printer Boy Hill ...................................................................... 510 

Iowa gulch ........................................................................... 511 

Head of California gulch ............................................................. 512 

Gold deposits ......................................................................... 513 

Placer deposits ....................................................................... 515 

Ore prospects in unexplored areas ..................................................... 518 

Mines and prospects outside the Leadville district ......................................... 519 

Northeastern region MonteCristo mine .............................................. 520 

Mount Lincoln Russia mi ue ......................................................... 521 

Mount Bross .......................................................................... 522 

Moose mine ................. , .................................................... 522 

Dolly Varden mine ............................................................... 523 

Buckskin canon ................. . .................................................... 524 

Phillips mine ..................................................................... 524 

Criterion mine ................................................................... 525 

Excelsior mine Colorado Springs mine ........................................... 526 

Dominion mine Sweet Home mine ............................................... 527 

Loveland Hill Fanny Barrett mine ................. . ............................... 528 

Between Mosquito and Horseshoe gulches ........... . ................................ 529 

Sacramento mine ................................................................. 530 


Other groups of mines Continued. 

Mines and prospects outside the Leadville district Continued. 

Crest of the Mosquito range 531 

London mine 532 

Peerless mine 533 

Western slopes of Mosquito range El Capitan mine 534 

Ten-Mile district 537 

Deposits in the Archean 538 



Manner of occurrence Why in Blue Limestone rather than in any other formation 540 

Composition of ores 543 

Carbonate ores . 543 

Chloride ores... 548 

Basic ferric sulphates 549 

Processes of alteration 550 

Agents of alteration 552 

Eclat i ve richness of galena and cerussite 553 

Outcrop deposits richer than those in depth 554 

Composition of vein materials 556 

Vein materials in general 557 

Kaolin and Chinese talc 560 

Lime and magnesia salts Barite 561 

Manganese 562 

Ores deposited as sulphides 562 

Mode of formation 565 

Indirect evidence 566 

Negative evidence 567 

Origin or source of the metallic minerals : 569 

Ascension or lateral secretion 569 

Source of metals 571 

Metallic contents of country rocks 574 

Baryta determinations Lead determinations 577 

Silver and gold determinations 579 

Possible contents of porphyry bodies 582 



Eruptive rocks 589 

Table I. Complete analyses 589 

Table II. Silica and alkali determinations 590 

Table III. Lead, zinc, cobalt, and barium determinations 591 

Remarks on preceding tables 591 

Table IV. Gold and silver determinations 594 

Limestones 596 

Table V. Complete analyses of dolomitio limestones 596 

Table VI. Lime, magnesia, and chlorine determinations 598 

Table VII. Serpentine and amphibole from dolomitic limestones 598 

Ores and vein materials 599 

Table VIII. Sand carbonates 599 

Table IX. Chloro-bromo-iodides of silver 600 


Tables of analyses and notes on methods employed Continued. 

Ores and vein materials Continued. 

Table X. Various ores, vein materials, and country rocks 602 

Table XI. Alteration products of porphyry (kaolin and Chinese talc) 603 

Table XII. Alteration products of galena and pyrite (basic sulphates) 606 

Table XIII. Miscellaneous alteration products 607 

Table XIV. Assays of ores, vein materials, and country rocks 608 





Leadvillo Its situation C14 

Location and output of its mines, Table I 615 

Ores Description of chi<-f ores Analyses of carbonate, chloride, and average ores Assays 

of ores from principal mines, Table II 616 

Smelting works Their location, cost of plant, Table III, and general arrangement 625 

Ore buying Its method 627 

. Sampling and sampling works 628 

Crushing and crushers used 629 

Assaying Apparatus and methods 632 



General considerations 636 

Statistics of Leadville smelters, Table IV C37 

Construction materials 641 

Fuels and flaxes 641 

Coke Analyses VII anil IX 641 

Charcoal 642 

Dolomites Analyses X to XIX 643 

Limestone Analysis XX 646 

Hematite Analysis XXI li-lfi 

Ore beds Their composition, Table V 648 

Smelting charges at individual smelters 649 

Resume" and general discussion of smelting charges 659 



Smelting plant in general Furnaces and blast apparatus. Table VI 659 

Smelting operations in general Drying, blowing-in, charging, barring-down, smelting of 
dusts, tapping of clay, matte and speiss, ladling-out of bullion, watching and blow- 
ing-out ^ 664 

Cost and profits of smelting 668 

Plant and operations of individual smelters G69 



Bullion Its sale Shipment, Table VII Analyses and assays Skimmings Losses Bull- 
ion capacity of smelters, Table VIII Bullion production, Table IX 692 


Products of smelting Continued. 

Slag Daily assays, Table X Specificgravity determinations Properties Complete analy- 
ses XXX to XXXII 698 

Chamber-dust Assays, Table XII; Analyses XXXIII and XXXIV Roasted dust, Analysis 

XXXV Fumes from Bartlett filter, Analysis XXXVI 711 

Speiss Analyses XXXVII and XXXVIII Assays, Table XIII 719 

Iron sows or salamanders Analysis XXXIX 722 

Mattes Analyses XL and XLI Assays, Table XIV 723 

Accretions Heartb accretions, Table XV Shaft accretions, Analysis XLII Peculiar accre- 
tions, Analyses XLIII and XLIV 7-25 



Reactions in the blast-furnaces 731 

Reactions of lead compounds 732 

Reactions of silver compounds 735 

Reactions of iron compounds 736 

Chemical discussion of the Leadville furnaces. 737 

Raw materials entering into charges 738 

Weight of blast 740 

Loss of weight of charges in different zones 741 

Chemical reactions of the different zones 744 

Conclusions 745 





PLATE I. Head of Iowa gulch Iowa amphitheater and Mount Sherman in the background. 

II. Leadville and the Mosquito Range 6 

III. South Park and eastern slopes of Mosquito Range, from Mount Silverheels 28 

IV. Archean phenomena 52 

FIG. 1. Face of cliff, Arkansas amphitheater. 

2. Bowlder, Buckskin amphitheater. 

3. Bowlder, Mosquito gulch. 

V. Red-cast beds (Camhrian). Contorted limestone (Upper Coal Measure) GO 

VI. Blue limestones (specimens showing ribbed structure) 64 

VII. Hornblende-porphyrite specimens 84 

FIG. 1. From Arkansas dike. 

2. From south wall of Buckskin gulch. 

VIII. Xevadite from Chalk Mountain 86 

FIG. 1. Micro-section of White Porphyry. 

2. Micro-section of Nevadite. 

3. Specimen of Nevadite. 

IX. Mount Lincoln Massive from Mount Silverheels 94 

X. Mosquito Range and Blue River Valley, looking north from Mount Lincoln 98 

XI. Summit of Mount Lincoln and north wall of Cameron amphitheater (upper end).. 112 
XII. North wall of Cameron amphitheater (lower end) 114 

XIII. South side of Buckskin gulch 128 

XIV. Cliffs on north side of Mosquito gulch (showing fault and intrusive masses of por- 

phyry) 132 

XV. Sacramento arch (looking across Big Sacramento gulch from Pennsylvania Hill) . 148 

XVI. North side of Horseshoe gulch (showing White Ridge and laccolite) 154 

XVII. Head of Four-Mile Creek (showing the "Horseshoe") 158 

XVIII. Sheep Mountain fold and London fault (south side of Horseshoe gulch) 164 

XIX. Rhyolite and porphyry, Chalk Mountain 196 

FIG. 1. Plan of eastern edge of mountain, showing quartzite and porphyry 
dike inclosed in rhyolite. 

2. North side of railroad cut, at south end of Chalk Mountain, showing 

porphyry cutting through sandstone. 

3. South side of same cut. 

XX. Micro-sections of porphyrite. Appendix A 336 

FIG. 1. Biotite-porphyrite (Type VI) from North Mosquito amphitheater. 

2. Hornblende-porphyrite (Type VII). 

3. Hornblende-porphyrite (Type V) from Buckskin gulch, sho.wing large 


4. Hornblende-porphyrite (Type V) showing small needles of hornblende. 





PLATE XXI. Micro-sections of audesite. Appendix A 354 

FIGS. 1, 2, and 4. Andesite from Buffalo Peaks. 

3. Zircon crystals in quartz grain of a Lincolu Porphyry. 


XXII. Vein phenomena, showing replacement action 420 

FIG. 1. Evening Star Incline. 

2. Glass-Peudery mine. 

3. Carbonate Incline. 

4. Forsaken Incline. 

XXI II. Circular furnace, smelter A 749 

XXIV. Heverberatory furnace and dust chamber, smelter A 749 

XXV. Flue arrangement, smoker A 749 

XXVI. Rectangular furnace, smelter B 749 

XXVII. Circular furnace, smelter D 7 49 

XXVIII. Bartlett smoke filter McAllister charcoal kiln 749 

XXIX. Rectangular furnace, smelter C 749 

XXX. Dust chamber, smeller C 749 

XXXI. Blast arrangement ami i-li'\ a t inn of works, smelter C 749 

XXXII. Furnace and ilnsl e lumber, smelter D 749 

XXXIII. Furnace aud dust ehambets, smelters F and M 749 

XXXIV. Dust chamber, smelter F 749 

XXXV. Square fnriuiees, smelter G 749 

XXXVI. Zones of temperature K leva t ion of smelter G ~ 749 

XXXVII. Circular furnace, smelter H 74H 

XXXVIII. Dust chamber, smelter H Jolly's spring-balance 749 

XXXIX. Assay furnace, smelter H 749 

XL. Dust chamber, smelter J 749 

XLI. Blake crushers 749 

XLII. Blowers Baker's and Root's 749 

XLI II. Assay implements 749 

XLIV. Smelter's implements 749 

XLV. Alden crasher Furnace and dust chamber, smelter I 749 

FlQ. 1. Sanidine from Nevadite 348 

2. Highland Chief mine 501 

3. Colorado Prince and Miner Boy mines 504 

4. Florence mine, Printer Boy Hill 510 

5. Taylor Hill 511 

6. El Capitau mine 536 



Title I 

List of Atlss sheets II 

Legeud Ill 

Central Colorado IV 

Mosquito Range : 

Topography V 

Geology. North half VI 

Geology. South half VII 

Geological sections. I VIII 

Geological sections. II IX 

Geological sections. Ill X 

Leadville and vicinity : 

Topography. East half XI 

Topography. West half XII 

Geology. East half XIII 

Geology. West half XIV 

Geological sections. I, west half <. XV 

Geological sections. I, east half XVI 

Geological sections. II, west half XVII 

Geological sections. II, east half -. XVIII 

Geological sections. Ill, west half XIX 

Geological sections. Ill, east half XX 

Geological sections. IV XXI 

Geological sections. V XXII 

Iron Hill : 

Geology anil mine workings XXIII 

Geological sections. I XXIV 

Geological sections. II XXV 

North Iron Hill : 

Geology and mine workings XXVI 

Geological sections XXVII 

Carbonate Hill : 

Geology and mine workings XXVIII 

Geological sections. I XXIX 

Geological sections. II XXX 

Fryer Hill : 

Geology and mine workings XXXI 

Geological sections. I XXXII 

Geological sections. II XXXIII 

Geological sections. Ill XXXIV 

Index to shafts on Leadville map XXXV 




SHEET VI. Blue section line BB should run through the summit of Mount Lincoln and thence to sum- 
mit of Mount Cameron, instead of direct to latter from point on east spur. 
VII. The line of the Mike fault should not bo continued south of Empire gulch. 
XII and XIV. The blue line, showing the course of the Starr ditch north of California gulch, 

has been omitted ; the names give an approximate idea of its position. 
XIII. Color on small block of Blue Limestone at Comstock tunnel (L-38) has been left out. 
XV. "Ditch" just west of Sequa shaft should be "Little Evans gulch." 
XXI. Section JJ, " Iowa gulch fault," should be "Iowa fault." 

XXIII. Parallel linings to denote " inclines" have been omitted on Silver Wave claim. 
XXV. Section FF, "California fault," should be " Dome fault." 
XXX. Section GG, " White Porphyry " color nnder drift east from upper shaft of Yankee Doodle 

mine, should have been that of " vein material." 
XXXI. Shaft " Carboniferous No. 7" should be "Carboniferous No. 1." 

Shaft "Little Chief No. 3" (southernmost) should be "Little Chief No. 5." 

Shaft "Little Pittsburgh No. 3" (near E, boundary line, and just north of dike), "No. 

3" left out. 

Shaft Climax No. 2 (near E, boundary line, and line of section), "No. 2" left out. 
XXXV. F-10 " Leavenworth " should be " Lawrence." 
M-5 "Beecher" should be "Belcher." 



The Mosquito Range, the study of whose geological structure formed a necessary basis for that of 
the ore deposits of the Leadville region, is the western boundary of the South Park, and has thus been 
considered from a topographical standpoint to form part of the Park Range. Geology shows, however, 
that in Paleozoic times the boundaries of the depressions now known as the Parks were formed by the 
Archean land masses of the Colorado Range on the east and of the Sawatch and its continuation to the 
north, the Park Range on the west, and that the uplift of the Mosquito Range did not occur until the 
close of the Cretaceous. 

Prior to this uplift the various porphyry bodies, which now form a prominent feature among the 
rock formations of the region, were intruded into the sedimentary beds deposited during Paleozoic and 
Mesozoic times, spreading out between the beds and sometimes crossing them, but being most uniformly 
distributed at the top of the Lower Carboniferous or Blue Limestone. It was in this limestone that the 
greater part of the ores were deposited, and the original deposition must have taken place after the 
intrusion of the porphyry aud before the uplift of the range. 

In the uplift of the range both eruptive sheets and sedimentary beds, with the included ore 
deposits, were plicated and faulted, aud by subsequent erosion an immense thickness of rocks has been 
carried away, laying bare the very lowest rocks in the conformable series; the outcrops are, however, 
frequently buried beneath what is locally called " wash," a detrital formation of glacial origin. In the 
Leadville region, owing to the reduplication caused by faulting, a series of outcrops of easterly dipping 
beds of the Blue Limestone are exposed beneath the wash, of which all are metalliferous and a consid- 
erable proportion carry pay ore. 


The principal ore deposits of Leadville occur, as above indicated, in the Blue Limestone and at or 
near its contact with the overlying bodies of porphyry. The ores consist mainly of carbonate of lead, 
chloride of silver, and argentiferous galena, in a gangue of silica and clay, with oxides of irou and 
manganese and some barite. These materials are mainly of secondary origin, nnd result from the altera- 
tion by surface waters of metallic sulphides. 

The study of these deposits has shown : 1, that they were originally deposited as sulphides, and 
probably as a mixture, in varying proportions, of galena, pyrite, and blende; 2, that they were de- 
posited from aqueous solutions ; 3, that the process of deposition was a metasomatic iuterchaugi 1 be- 
tween the materials brought in by the solutions and those forming the country rocks, consequently 
that they do not fill pre-existing cavities; 4, that the ore currents from which they were deposited 
did' not come directly from below, but were more probably descending currents ; and 5, that these 
currents probably derived the material of which the ore deposits are formed mainly from the por- 
phyry bodies which occur at horizons above the Blue Limestone. 


Inasmuch as the ore currents did not come directly from below, it is not advisable to search for 
ore below the Blue Limestone horizon. This horizon, however, should be thoroughly prospected, and 
the maps and sections show its probable position in the as yet unexplored areas ; the explorations, 
moreover, should not be confined to the upper surface of this limestone, but carried into its mass 
wherever there are indications of ore, and especially along the contact of transverse bodies of Gray 
Porphyry. The probabilities are that very considerable bodies of ore remain as yet undiscovered, and 
the most promising areas for prospecting are indicated. It is also probable that as the distance from 
the surface increases the ores will be found less altered, and that they will therefore be lesseasily 
reduced by the smelting processes now employed. 

The petrography of the district is treated by Mr. Whitman Cross in Appendix A. The results of 
chemical investigation aud the methods of research are given in Appendix B by Mr. W. P. Hillebrand, 
and in Appendix C Mr. Guynrd has given a memoir on lead smelting as conducted at Leadville, show- 
ing the character of the plant, the composition of ores, fluxes, aud furnace products, aud discussing the 
reactions which take place in the blast furnaces. 










Topographical description. The city of Leadville is situated in the county 
of Lake, State of Colorado, on the western flank of the Mosquito Range, 
at the head of the Arkansas Valley. Its exact position is in longitude 
106 17' west from Greenwich and 39 15' north latitude. Its mean 
elevation above sea-level is 10,150 feet, taken at the court-house, in the 
center of the city. 1 

The most striking feature in the topographical structure of the Rocky 
Mountains in Colorado is, as is well known to those familiar with western 
geography, the fact that it consists of two approximately parallel ridges, 
separated by a series of broad mountain valleys or parks. 

The easternmost of these uplifts, the Colorado or Front Range, rises 
abruptly from the Great Plains, which form its base at 5,000 to 6,000 feet 
above the sea-level, to a crest of 13,000 to 14,000 feet. It is deeply scored 
by narrow, tortuous gorges, worn by mountain streams, whose clear waters 
debouch upon the plains and become absorbed in the sluggish, turbid 
currents of the Platte and Arkansas Rivers. The trend of the range is 
due north and south, its highest portions being mostly included within the 

'The datum point from which the levels of the map of Leadville were reckoned is the threshold 
of the First National Bank, a stone building at the southeast corner of Harrison avenue and Chestnut 
street. The altitude of this point, as determined by connection by levels with the bench-marks of the 
Denver and Rio Grande Railroad, is 10,135.55 feet ; by levels with the bench-marks of (he Colorado 
Central Railroad, 10,113 feet ; by depression angles from the top of Mount Lincoln, 10,112 feet. As a 
mean, the contour passing through it is assumed to be 10,125 feet, greater weight being given to the 
first figure, since the leveling by which it was arrived at was probably more carefully done than in 
the case of the other two. A level-line had been run from Fairplay to the top of Mount Lincoln by the 
members of the Haydeu Survey in 1872. 



boundaries of the State, beyond which at either end it becomes gradually 
lower, and disappears as a topographical feature beneath the plains. To 
the west of this range lie the mountain valleys of the North, Middle, South, 
and San Luis Parks, in Colorado, and the Laramie Plains, in Wyoming, 
each of which possesses the same general feature of being nearly completely 
encircled by mountain ridges. On the other hand, each has distinct topo- 
graphical features of its own, which need not be entered upon here. 

Beyond the parks on the west, and separating them from the great 
basin of the Colorado River, is a second mountain uplift, to which the gen- 
eral name of Park Range has been given. It has by no means the regular 
structure of the Colorado Range, but is made up of a series of short ranges 
en e'chelon, from which offshoots connect with the latter, terming the ridges 
which separate the different park basins. In the latitude of Leadville this 
western uplift consists of two distinct ranges, the Mosquito or Park Range 
the latter being the name given in the Hay den atlas of 1877, probably 
because it forms the boundary of the South Park and the Sawatch Range, 
which forms the water-shed between the Atlantic and Pacific waters. 

The Mosquito Range is a narrow, straight ridge, about eighty miles in 
length, trending a little west of north, and is characterized by long, regular 
slopes scored deeply by glacial gorges on the east toward South Park and 
by an abrupt irregular inclination on the west towards the Arkansas Valley. 

The Sawatch Range, on the other hand, is a broader, oval-shaped 
mountain mass, divided by the deep gorges of its draining streams into a 
series of massives and wanting the continuous ridge structure of the Mos- 
quito Range. In this respect, as in its geological composition, which is the 
determining cause of the difference of its topographical forms, it resembles 
the Colorado Range. The culminating points of each range have a remark- 
ably uniform elevation of about fourteen thousand feet above sea-level. 

Between the two ranges lies the valley of the Upper Arkansas, a merid- 
ional depression 60 miles in length and about sixteen miles in width, measured 
from the crest of its bounding ridges. Its direction is parallel to that of 
the Mosquito Range, being a little east of south in its mean course, though 
more nearly north and south towards its head. From its southern end the 
Arkansas River, after receiving the waters of the South Arkansas, bends 


sharply to the east and cuts through the southern continuation of the Mos- 
quito and Colorado Ranges in deep canon valleys, the last well known to 
tourists as the Royal Gorge. About midway in the Upper Arkansas Valley 
the present bed of the stream is confined within a narrow rocky canon, 
called from the prevailing rock of the surrounding hills Granite Canon. 
Both above and below this canon the foot-hills of the bordering ranges 
recede again, leaving a valley bottom from six to ten miles- in width. But 
little of this area is occupied by actual alluvial soil, its surface consisting 
mostly of gently sloping, gravel-covered terraces. In the area above the 
canon, which is about twenty miles long, the eye is at once arrested by its 
basin form. In the center is a relatively wide stretch of meadow land imme- 
diately adjoining the river, on either side of which mesa-like benches slope 
gently up to the foot-hills of the mountains, three or four miles distant, 
which rise abruptly from these terraces in broken, irregular outlines. The 
suggestion thus offered by its basin shape and terrace-like spurs that this 
portion of the valley was once filled by a mountain lake is confirmed, as 
will be seen later, by the geological facts developed during the present 

On the tipper edge of one of these terraces, on the east side of the val- 
ley, is situated the city of Leadville. From the north bank of California 
gulch it extends along the foot of Carbonate hill to the valley of the east 
fork of the Arkansas, covering, with its rectangular system of streets and 
contiguous smelting works, an area of nearly 500 acres, while on the hill 
slopes immediately above are situated the mines which constitute its wealth. 
On Plate 1 1 is given the reproduction of a photograph of the city, taken 
from a point in its western outskirts on Capitol Hill ridge, near the junc- 
tion of the two branches of the Denver and Rio Grande Railroad and about 
west of the Harrison smelter. Although the plate leaves much to be de- 
sired in point of distinctness and the shape of the mountain spurs back of 
the town are necessarily obscured by foreshortening, it serves to give a 
general idea of the city and its surroundings. The square building with 
cupola, on the extreme left, is the court-house, back of which the wooded 
ridge in the middle distance is Yankee Hill; a similar building to the right 
toward California gulch is the high school. The chimney in the middle is 


that of the Harrison Reduction Works, to the right of which is the Tabor 
mill. The slopes immediately back of the town are those of Carbonate hill, 
beyond which is seen the round summit of Ball Mountain, with Breece hill, 
as a wooded spur, extending northward from it, Still farther back the ridge 
slopes up in apparent continuity to Dyer Mountain, the highest point on the 
sky-line To the left of Dyer Mountain is Mount Evans, 6^ miles distant in 
a straight line,.and on its right is Mount Sherman, forming the eastern walls 
of Evans and Iowa amphitheatres respectively. On a clear day the outlines 
of rock formations on these walls may be very distinctly seen. 

Routes of approach. The approach to Leadville, as may be seen from the 
above brief sketch of its topographical situation, was extremely difficult be- 
fore the development of its wealth had led to the building of railroads. 
Three routes of travel were available. The middle one, or that most used 
by travelers in coming from Denver, crossed the Colorado Range near the 
South Platte Caflon, at an elevation of 10,001) feet, and skirting the northern 
rim of South Park, through the mining town of Fairplay, crossed the Mos- 
quito Range at Mosquito pass opposite Leadville at an altitude of 13,600 
feet, or, making a detour of ten or twelve miles to the southward, at Weston's 
pass, whose summit is only 12,000 feet above the level of the sea. This gen- 
eral route the Denver and South Park Railway follows, winding up the nar- 
row and tortuous gorge of the South Platte and passing over Kenosha pass 
at the head of its north fork into South Park; to cross the Mosquito Range, 
however, it is obliged to make a longer detour to the southward and pass 
down the valley of Trout Creek, a tributary of the Arkansas, which, heading 
on the east side of the Mosquito Range, debouches into the Arkansas Valley 
at Buena Vista, 40 miles south of Leadville. 

The southern route, before the time of railroads, generally crossed the 
Colorado Range at the Ute pass above Colorado Springs, and, traversing 
the lower end of South Park, passed into the Arkansas Valley either at 
Trout Creek or at Weston's pass. The Denver and Rio Grande Railway, 
however, has located its line a triumph of engineering skill directly 
up the valley of the Arkansas, which it follows through canons and gorges 
that before were practically impassable. 


Evans Amphitheatre. 

Dver Ml. 


Leadvllle ant 


Iowa Amphitheatre. 

Alt. Sheridan. 

JMosquito Range. 


The northern route starts from Golden, near Denver, and, following up 
the canon of Clear Creek, crosses the Colorado Range at an altitude of 
12,000 feet, either by the Argentine or by Loveland's pass It then crosses 
the southern edge of Middle Park along the valley of Snake River and 
bends southward up the valley of Ten-Mile Creek, having thus gone around 
the northern end of the Mosquito Range. After crossing the relatively low 
divide of Fremont's pass (11,300 feet), it reaches Leadville by descending 
the east fork of the Arkansas. At either end of this route railroads are 
already built, namely, up the valley of Clear Creek to Georgetown, and 
from Leadville across Fremont's pass down Ten-Mile Valley to its junction 
with the Blue. But the advisability of completing the connecting link at 
such an altitude, in practical competition with the two already existing lines, 
seems, under present conditions of development to be somewhat doubtful. 

Discovery of the precious metals. The discovery of the Leadville deposits 
presents so striking a picture of the life of the pioneer miner in the West, 
and of the large element of chance connected with it, that it seems proper 
to give its history with all the fullness of detail which the somewhat imper- 
fect data obtainable will allow. 

The earliest known exploration of the valley of the Upper Arkansas 
was that made by the expedition of Fremont in 1845. In his second expe- 
dition, in 1842, he had aimed at tracing the Arkansas River to its source, 
but, unwittingly leaving the main stream, had followed up the Fontaine qui 
bouille, now called Fountain Creek, probably passing near the present site 
of Denver, and struck into the mountains at some point nearly opposite 
that place. In 1845, however, as indicated by General Warren, he prob- 
ably entered the mountains near where Canon City now stands, and crossed 
the southern end of South Park, reaching the Upper Arkansas Valley 
through the valley of Trout Creek. Thence, following the Arkansas to its 
head, he crossed what was then called Utah pass and descended Eagle or 
Piney River to its confluence with the Grand or Blue. It seems proba- 
ble, therefore, that the name of Fremont's pass, which is given to that of 
Ten-Mile Creek, would have been more appropriately applied to the Ten- 
nessee pass, which divides the Eagle River from the head of the Arkansas. 


There is little doubt that this striking valley was afterward visited by 
trappers and individual explorers, but of such visits no record is left so far 
as is known to the writer. This region, like that of the parks, formed part 
of the debatable ground between the tribes of Arapahoes and Utes, who 
were constantly at war with each other and who made excursions to these 
mountain valleys simply for the purpose of hunting and without any per- 
manent occupancy. 

During the summer of 1859, at the time of the great Pike's Peak excite- 
ment, a continuous stream of emigrant wagons stretched across the plains, 
following up the Arkansas River to the base of Pike's Peak. As is gener- 
ally the case in such mining rushes, the golden dreams of a large portion 
of those attracted by the marvelous stories of the wealth that existed in the 
streams issuing from the mountains were never realized. Many of the 
wagons that had crossed the plains in the early summer, carrying the tri- 
umphant device " Pike's Peak or bust," returned later over the same route 
with this device significantly altered to "Busted." The more adventurous 
and hardy of these pioneers, although disappointed in their first anticipa- 
tions, pushed resolutely up through the rocky gorges towards the sources 
of the streams. Some of these found gold in Russell gulch, in the valley 
of Clear Creek, where the first mining developments were made within the 
State and where now stand the flourishing mining towns of Central City 
and Black Hawk. Others wandered across the Colorado Range into South 
Park, and found gold-bearing gravel deposits on its northern border, in 
Tarryall Creek and on the Platte in the neighborhood of Fairplay. This 
is, as far as can be learned, the extent of the explorations made in 1859. 

In the early spring of 1860 several small parties crossed the second 
range into the Arkansas Valley. Among the number were Samuel B. Kel- 
logg, now justice of the peace at Granite, and H. A. W. Tabor, later mill- 
ionaire and lieutenant governor of the State of Colorado. Mr. Kellogg 
had already had an experience of ten years in placer mining in California 
when he came toColoradoin 1859. In February, 1860, he started with Tabor 
and his family, their wagon being the first that ever went as far as the mouth of 
the Arkansas. They pushed up the valley and about April 1 settled down at 
the site of the present town of Granite, about eighteen miles below Lead- 


ville. Here, having discovered gold in Cash Creek, whose placer deposits 
are worked even at the present day, they whipsawed lumber to make sluices 
for washing its gravels. A few days after their arrival news was brought 
to them of the discovery of gold in California gulch: Two parties of prospect- 
ors had, it seems, already preceded them, though their route is unknown. 
Foremost among their names are those of Slater, Currier, Ike Rafferty, 
George Stevens, Tom Williams, and Dick Wilson, from the last of whom 
many of the following facts were obtained : The first hole dug in California 
gulch was about two hundred feet above the site of the present Jordan tun- 
nel, the second just below the present town of Oro. Owing to the richness 
of the ground and the number of the persons present, gold was discovered 
at an unusual number of points, and 14 discovery claims of 100 feet each 
were located. Kellogg and Tabor met the prospectors at the mouth of 
Iowa gulch, as they returned from locating the discovery claims, and 
agreed to prospect that gulch. They returned to Cash Creek for provis- 
ions, and went finally to California gulch on the 26th of April, 1860, as 
Iowa gulch had yielded little fruit to their labors the geological reasons 
for which will be explained later. 

In spite of the difficulties of communication in this wild region, the news 
of the rich discovery of gold spread with amazing rapidity. The day after 
their arrival 70 persons came into the gulch from the Arkansas Valley; by 
July it was estimated that there were 10,OQO persons in the camp. It is said 
that $2,000,000 worth of gold was taken out during the first summer. Prob- 
ably considerable deductions may be made from this estimate for the exag- 
geration that fills men's minds in moments of such excitement. The record 
of claims located, however, shows enormous activity in mining during this 
summer. In California gulch alone, 339 claims, 100 feet in width, were 
located. Single individuals are said to have carried away from $80,000 to 
$100,000 each as the result of their first summer's labor. Tabor and Kel- 
logg worked their own claims and made about $75,000 in sixty days. The 
total production of the placer claims is generally stated at from $5,000,000 
to $10,000,000, but a more conservative estimate places it at from $2,500,000 
to $3,000,000. The climax was soon reached, and after the first year the 
population of this new district, whose post-office was then known as Oro 


City, rapidly decreased, until within three or four years the thousands had 
dwindled into hundreds. Kellogg, with the restless spirit of the western 
prospector, wandered away in the early part of the summer into the San 
Juan region and did not return. Tabor started the solitary store in the 
place, his wife being at the time the only person of her sex in the camp. 
When the product of the placers had gradually decreased and the prosperity 
of the camp was at its lowest ebb, he moved across the range to Buckskin 
Joe, which was then enjoying a fitful prosperity from the rich developments 
of the Phillips mine ; but returned later, when the discovery of vein gold 
in the Printer Boy mine revived for a time the waning prosperity of the 

Development of mines. In 1861 a ditch was built from Evans gulch across 
the head of California gulch, by means of which sluice mining was carried 
on, but owing to the great cost of supplies, which had to be brought in on 
the backs of animals, only the very richest gravels could be worked with 
profit, and at that time little attention was paid to vein deposits. Among 
the early miners it is probable that few if any suspected the existence of 
the real mineral wealth that the region contained, although they were much' 
annoyed in their working by worn, iron-stained fragments of heavy rock, 
which they had to throw out by hand from their sluices, the water not having 
sufficient force to carry them down. 

Report says that in August, 1861, C. M. Rouse and C. H. Cameron, 
of Madison, Wis., "struck carbonates," of which a small quantity was 
shipped to George T. Clarke, of Denver ; and that samples which he sent 
to Chicago yielded by assay 164 ounces of silver to the ton. The Washoe 
Mining Company is said to have been formed on the strength of these dis- 
coveries, but no work was done upon the claims, whose location, if they 
really existed, is now unknown. 

In June, 1868, the first gold vein, called the Printer Boy, was discov- 
ered by Charles J. Mullen and Cooper Smith, who were prospecting for J. 
Marshall Paul, of Philadelphia; and in August the Boston and Philadelphia 
Gold and Silver Mining Company of Colorado was organized, and a stamp 
mill was built at Oro, in California gulch, to treat the ore from this vein. A 
very considerable amount of gold is said to have been obtained from it, 


though it is difficult to obtain actual data as to its production. Estimates 
place its total yield at $603,000 to $800,000. The "5-20" vein was also 
opened at this time on the opposite side of the gulch, and also an extension 
of the Printer Boy, called the Lower Printer Boy. The working of these 
mines, which was earned on more or less continuously until 1877, imparted 
at times a fitful prosperity to the region. Meanwhile the location of the 
town of Oro had been frequently changed. It was first scattered along 
California gulch, then concentrated at the mouth of the gulch, near the 
present city of Leadville, and later moved up to the vicinity of the stamp 
mill, which still stands among the few cabins to which the name of Oro City 
is yet applied. 

During this time the Homestake mine in the Sawatch Range, near 
Homestake Peak, opposite the head of the Arkansas, had been opened and 
was yielding rich silver ore. In 1875 a smelter was built at Malta, west of 
Oro, to treat the ore from this mine and from others which it was expected 
would be developed in that region. This smelter, like so many others built 
before any permanent production could be counted on for its supply, has 
never been successful. 

To Mr. A. B. Wood and his associate, Mr. W. H. Stevens, both experi- 
enced and scientific miners, is due the credit of being the first to recog- 
nize the value of the now famous carbonate deposits of Leadville. Mr. 
Wood came to California gulch first in April, 1874, to work the Star placer 
claim. While examining the gravel in the gulch he was struck by the 
appearance of what the miners call "heavy rock," some of which he 
assayed. His specimens were not rich, yielding only 27 per cent, lead 
and 15 ounces silver to the ton; but the matter seemed to him worthy 
of investigation He put prospectors at work to find the croppings of the 
ore deposits, and in June, 1874, the first " carbonate-in-place " was found 
at the mouth of the present Rock tunnel, on Dome hill About the same 
time ore was discovered in a shaft sunk by Mr. Bradshaw near the bed of 
the gulch on the present Oro La Plata claim ; but it is maintained by some 
that this ore was not in place, but simply "wash," accumulated from the 
abrasion of the adjoining croppings. Prospecting was quietly continued by 
Mr. Wood, but no claims were taken up, as the old placer claims which, 


though abandoned, would still be in force for another year covered all the 
ground adjoining the gulch. Meanwhile he studied the occurrence of the 
mineral and the outcrops of the limestone on either side of California gulch. 
In the spring of 1875 he took Mr. Stevens and Professor H. Beeger, the latter 
then in charge of the Boston and Colorado Smelting works at Alma, to 
Iron and Dome hills, and showed them in the forest that then covered the 
slopes the outcrops, respectively, of the Lime, Rock, and Dome claims. Dur- 
ing this and the following summer the principal claims which constitute the 
valuable property of the Iron Silver Mining Company were located by 
Messrs. Wood and Stevens in the interest of Detroit parties. The first ore 
was extracted from the Rock mine, where a large mass of hard carbonate 
formed a cliff outcrop on the side of California gulch. This ore was rich 
in lead, but ran very low in silver. During the summer of 1876 ore was 
first taken from the croppings of Iron and Bull's Eye claims, and some rich 
assays, as high as 600 to 800 ounces to the ton, were obtained from it. 

The first working tests of Leadville ore were made by Mr. A. R. Meyer, 
a graduate of European mining schools, who first came to California gulch 
in 1876 from Alma, acting as agent for the St. Louis Smelting and Refining 
Company. In the fall of that year he shipped 200 to 300 tons of ore, princi- 
pally taken from the Rock mine, by wagon to Colorado Springs, and thence 
by rail to St. Louis. The freight to Colorado Springs cost $25 per ton 
and the ore averaged only seven ounces in silver to the ton ; it contained, 
however, 60 per cent, lead, and in spite of the high cost of freight yielded a 
profit, owing to the high price of lead (seven cents a pound) then ruling. 
It having thus been proved that Leadville ore could be worked at a profit, 
prospecting was vigorously carried on, the next discovery being that of the 
Gallagher Brothers on the Camp Bird claim, supposed at that time to be the 
northern continuation of the Iron-Lime outcrop. This discovery was made 
late in the fall of 1876, and the claim now forms part of the property of 
the Argentine Mining Company. During this winter the Long and Derry 
mine was discovered by two prospectors of these names, who still own the 
mine and have become wealthy from its product. During the spring and 
summer of 1876 discoveries were made along what was then known as the 


second contact, on Carbonate hill, the Carbonate and Shamrock mines being 
the first to yield considerable quantities of pay ore. 

In the following years the famous ore bodies on Fryer hill were discov- 
ered by a singular accident. At this point there is no outcrop, the whole 
surface of the hill being covered to an average depth of 100 feet by detri- 
tal material. Tradition has it that two prospectors were "grub-staked," or 
fitted out with a supply of provisions, by Tabor, half of all they discovered 
to belong to him. Among the provisions was a jug of whisky, which proved 
so strong a temptation to the prospectors that they stopped to discuss its 
contents before they had gone a mile from town. When the whisky had 
disappeared, though its influence might probably have been still felt, they 
concluded that the spot on which they had thus prematurely camped was 
as good a one to sink a prospecting hole on as any other. At a depth of 25 
or 30 feet their shaft struck the famous ore body of the Little Pittsburg 
mine, the only point on the whole area of the hill where rock in place comes 
so near the surface. Discoveries rapidly multiplied in this region; immense 
amounts of ore were taken out, and the claims changed hands at prices 
which advanced with marvelous rapidity into the millions. A half interest 
in one claim which was sold one morning for $50,000, after being trans- 
ferred through several hands, is said to have been repurchased by one of 
the original holders for $225,000 on the following morning. 

The foundation of Mr. Tabor's wealth was laid in the first discovery 
on Fryer hill, but its amount was materially increased in a singular way. 
When the fame of the rich discovery of Fryer hill had already become 
known at Denver, the wholesale house from which he was in the habit of 
buying his provisions commissioned him to buy for them a promising 
claim. On his return to Leadville, in accordance with this agreement, he 
purchased on their account, for the sum of $40,000, the claim of a some- 
what notorious prospector known as Chicken Bill, on what is now Chryso- 
lite ground. Chicken Bill, in his haste to realize, had not waited till his 
shaft reached rock in place, but had distributed at its bottom ore taken 
from a neighboring mine, or, in the language of the miners, he had " salted " 
his claim. After the bargain with Tabor had been concluded he could not 
resist the temptation of relating to a few of his friends the part he had 


played in the transaction. The report of what he had done thus reached the 
ears of Mr. Tabor's Denver correspondents before he himself arrived to 
deliver the property, when they not unnaturally declined to receive it, and 
Mr. Tabor was obliged to keep it himself. He, with his associates, under 
the title of Tabor, Borden & Co., afterward bought some adjoining claims 
and developed their ground, from which they are said to have taken out 
in the neighborhood of $1,500,000, and afterward to have sold their prop- 
erty to the Chrysolite Company for a like sum. 

In the spring of 1877, under Mr. Meyer's direction, the first smelting 
furnace was erected at Leadville by the St Louis Smelting and Refining 
Company, now known as the Harrison Reduction Works, and others fol- 
lowed in rapid succession. 

Growth of the city. The nucleus of the present city of Leadville consisted 
of a few log houses scattered along the borders of the California gulch 
below the Harrison Reduction Works. In the spring of 1877 a petition 
for a post-office was drawn up by Messrs. Henderson, Meyer, and Wood, 
which necessitated the adoption of a name for the new town. Mr. Meyer 
proposed the names of Cerussite and Agassiz, both of which were rejected 
as being too scientific. Mr. Wood proposed the name of Lead City, to 
which Henderson objected that it might be confounded with a town of the 
same name in the Black Hills, and the name of Leadville was finally 
adopted as a compromise. The rapidity of the growth of this city borders 
on the marvelous. In the fall of 1877 the population of Leadville was esti- 
mated at about two hundred persons. The business houses of the town 
were a 10 by 12 grocery and two saloons. In the spring of 1878 a corpo- 
ration was formed, which was continued for six weeks, when the town's 
growth justified its transformation into a city of the second class, Mr. W. 
H. James being the first mayor and John W. Zollars city treasurer. Within 
two years Leadville grew to be the second city in the State, with 15,000 
inhabitants and assessable property of from $8,000,000 to $30,000,000. In 
1880 it had 28 miles of streets, which were in part lighted by gas at an 
expense of $5,000 per annum. It had water-works, to supply all the busi- 
ness portion of the city, having over five miles of pipe laid. It had 13 
schools, presided over by 16 teachers, and an average attendance of 1,100 


pupils; a high school, costing $50,000; five churches, costing from $3,000 
to $40,000 ; and three hospitals, in one of which 3,000 patients were treated 
during the year. In 1880 $1,400,000 were expended in new buildings and 
improvements. It had 14 smelters, with an aggregate of 37 shaft-furnaces, 
of which 24 were in active operation during the census year, and its produc- 
ing mines may be roughly estimated at 30. 

Production. The amount that is annually added to the metallic wealth of 
the world by the Leadville district, the productive area of whose deposits 
as at present opened may be estimated at about a square mile, is truly 
remarkable. Its annual silver product alone is greater than that given by 
official estimates for any of the silver-producing nations of the world out- 
side of the United States except Mexico. Its lead product, on the other 
hand, though frequently neglected in estimating the total value of its out- 
put, is nearly equal to that of all England, and, of other nations outside of 
the United States, it is only exceeded by that of Spain and Germany. 

In the magnitude of its product Leadville has been only surpassed in 
the United States by the famous Comstock lode in the Washoe district of 
Nevada, and the surprising rapidity of its development in the few years of 
its existence has been even more remarkable than that of the latter, which 
produced forty- eight millions of gold and silver during the five years suc- 
ceeding its discovery. The third district of comparable importance in the 
magnitude of its product from a comparatively restricted area is the Eureka 
district of Nevada, which, according to Mr. Curtis, has, in the first fourteen 
years of its existence, produced sixty millions of gold and silver and 225,000 
tons of lead. 1 

Owing to the want of any general law compelling producers to fur- 
nish an exact and sworn statement of the amount of their annual product, 
it is impossible to obtain anything more than an approximate estimate of 
the metallic production of a mining district like Leadville. Such an esti- 
mate varies necessarily in the closeness of its approximation, with the care 
with which it is made, with the accuracy with which the records of indi- 
vidual mines and smelters have been kept, and with the readiness shown 

1 J. S. Curtis, Silver-lead Deposits of Eureka. Washington, 1884. 



under varying circumstances to furnish these records to those who may be 
gathering statistics. 

The most trustworthy estimates of production are those that were 
obtained for the year ending May 31, 1880, by those engaged in collecting 
statistics of the production of the precious metals for the Tenth Census. 
This is due to the fact that not only was the force of experts sufficient 
to visit personally all the important mines and smelting works, but the 
law gave them the authority to demand, if necessary, an accurate tran- 
script of their records, and the data thus gathered were subjected to a crit- 
ical analysis during compilation by those technically familiar with the 
various branches of mining industry. Moreover, it was a most favorable 
epoch in the development of the district for obtaining an accurate record, 
since the larger mines were being systematically worked, the record of 
their product was kept with relative accuracy, and as yet but little ore was 
shipped out of the district for reduction and thus rendered difficult to 

The Census figures of production for this period are as follows : 

Leadville productg during censuf year, 187!>-'SO. 

Grose weight. 





I. Ore extracted 


152, 241 

138, 110, 797 
25, 657, 921 



10, 603, 331 
9, 717, 819 
8, 053, 948 

329, 763. 5 
302, 224 
250, 478 



II. Ore smelted 

III. Bullion produced by 
Leadville smelters. 

25, 608, 212 

In the above table, I gives the amount of ore extracted from the vari- 
ous mines during the year and the contents of the same in silver and gold, 
as determined by assajr at the mines. 

II gives the amount of ore smelted during the year and its assay 
value in silver and gold, including that sent out of the district for reduction, 
as determined by the returns from smelters and sampling works. 

III gives the bullion produced during the census year by the smelt- 
ers situated at Leadville and its contents in lead, silver, and gold. 


It thus appears that the Leadville ores contained during the year an 
average of 69J ounces of silver per ton, and that the bullion produced 
therefrom contained an average of 285 ounces of silver per ton. The 
apparent discrepancy in the amount of gold given under the various heads 
may arise in part from the fact that it is generally present in such minute 
quantities in the ore that the assayers at the mines do not always make an 
estimate of it, and in part from small lots of gold-bearing ore either from 
Leadville itself or from adjoining districts that have escaped notice in making 
up the returns from mines, or in segregating outside ore in returns from 
sampling works and smelters. It was not possible to obtain an accurate esti- 
mate of the average percentage of lead contained in all the ores extracted. 
It appears, however, from data obtained from the eight principal smelters 
running at that time that the average yield per ton of ores treated by them 
during the year was 398.8 pounds or 19.94 per cent, of lead bullion, con- 
taining 65.64 ounces or 0.225 per cent, of silver. 

The various newspapers of Leadville have published monthly state- 
ments of the bullion product of the district, upon which the annual official 
statements made by the Director of the Mint and other estimates of the 
product of the district have been based. These figures often bear internal 
evidence of incompleteness or inaccuracy, and from want of any evidence 
of the relative care with which they have been made, it is difficult to know, 
in cases of discrepancy between them, which is the most trustworthy. 
Nevertheless, in the absence of any other complete data, these must be 
assumed as the nearest approximation available. 

The following table of the product of the district, since the discovery 
of silver-lead deposits, has been compiled from these sources, using mainly 
the figures of the Leadville Herald, which have been the most continuously 
collected and published. In the case of shipments of ore to be reduced 
outside the district, of which only the price received is in many instances 
given, the weight of the metals contained in these shipments has been 
assumed arbitrarily to average the same as those in which the relative 
weights are known, which evidently cannot give the exact amount in every 
case, but which would be probably as nearly correct as an arbitrary 
assumption of probable averages for each year. The value of the total 




product is calculated according to the mint valuation ($1.2929 per ounce 
of silver), which, as is well known, is in the case of silver considerably 
higher than the fluctuating market value, and increases the value given for 
the total product by about seven million dollars above that which would 
be obtained by using the market value, if it were possible to obtain it in 
each case. The price of lead is assumed at 4 cents a pound as an aver- 
age for the whole period involved : 

Production of LeadriUe mines from Ib77 /o 18 J 4, inrlii*ire. 






77, 197 


42, 089, 722 
0. 012, 644 



203, 831 

102. 867 

93, 319, 399 

74, 358, 395 

Shipped out of the district. . - 

103, 022 


51, 102, SCO 

1, 5S9, 283 


278, 231, 825 

95, 864, 738 

In the time that has elapsed since the census year, although, owing 
partly to decline in value of the metals and partly to a lower average tenor 
of the ore, the total value of the annual product has decreased, the amount 
of ore extracted from the mines of the district has very- considerably 
increased, this having been in the census year (1879-1880) 152,241 tons, 
and in the year 1884, according to the report of the Director of the Mint, 
232,000 tons. 




The simplest expression of the geological structure of the Rocky 
Mountains in Colorado is that of two approximately parallel uplifts or series 
of ridges of Archean rocks, upon whose flanks rest at varying angles a 
conformable series of sedimentary formations extending in age from the 
earliest Cambrian to the latest Cretaceous epochs, the latter being locally 
overlaid by unconformable Tertiary beds. 

The eastern uplift is generally known as the Colorado or Front Range 
and the western as the Park Range, the series of depressions or mountain. 
valleys between them having received the name of parks. 

The most prominent fact thus far recognized in the geological history 
of this region is that a great physical break or non-conformity in the strata 
is found between the Cretaceous and Tertiary formations; in other words, 
that at this period occurred the great dynamic movement which uplifted 
the Rocky Mountain region essentially into its present position. As the 
beds of the Paleozoic and Mc.sozoic systems have been thus far found to 
be practically conformable throughout the region, it may be assumed that 
no important dynamic movement took place during these eras, and that 
deposition went on continuously, except when continental elevations of the 
whole region may have caused a temporary recession of the waters of the 
ocean for a limited period, and thus produced a gap or gaps in the geolog- 
ical series . without causing any variation in angle of deposition in the at 
present successive beds. 



Eastern uplift. The Colorado or Front Kange is the move extensive and 
more important of the two Archean uplifts, and along its eastern flanks is 
exposed, by the denudation of the overlying Tertiary formations, an almost 
continuous fringe of upturned Paleozoic and Mesozoic beds. 

The most significant geological fact to be observed in connection with 
these exposures of upturned beds is that the formation which is immediately 
adjacent to the Archean varies from place to place. At one point Triassic 
beds, sloping away at varying angles from the flanks of the mountain, rest 
directly upon the Archean beds; at another point the lower beds of the Cre- 
taceous; at still another, and this more rarely, the Carboniferous limestones 
are exposed resting against the Archean, while above them, always con- 
formable, are found the Triassic, Jurassic, and Cretaceous formations as 
one follows the section in an ascending geological sense. At one or two 
points only along the eastern flanks Silurian beds are exposed beneath 
the Carboniferous. 

It has been customary with many of the early geological explorers 
to consider the uplift of these mountain ranges to be that of a simple anti- 
clinal fold in the sedimentary strata, which once arched over the underlying 
nucleus of crystalline rocks; this was once considered the typical structure 
of a mountain range. In practical field geology, however, it is found that 
the symmetrical form resulting from this typical structure of mountain 
range is one of the rarest occurrences, at least in the Rocky Mountain 
region.' The one great instance of such a perfect anticlinal range is that ot 
the Uinta Mountains, which presents exceptional features distinguishing it 
from the majority of mountain ridges of the Rocky Mountain system ; this 
has a peculiarly normal anticlinal structure in the first place, and in the 
second place its trend is east and west, whereas all the other great mount- 
ain ridges of the Cordilleran system have a direction varying between north 
and south and northwest and southeast. 

The facts just noticed with regard to the sedimentary beds which rest 
against the eastern flunks of the Rocky Mountains, it will be readily seen, 
exclude the possibility of the typical anticlinal structure above mentioned. 
If we suppose a conformable series of sedimentary beds to have been folded 
into a long anticlinal fold and the crest of this fold subsequently planed 


off by erosion, so that the core of the fold is exposed, the projection or hori- 
zontal section made thus by the planing off of its crest would necessarily 
show a continuous line of outcrops along either side of the axis of the fold, 
in which the lowest bed of the conformable series would invariably be seen 
at the contact of the underlying rocks which, when these beds were depos- 
ited, formed the floor of the then existing ocean. In other words, if the 
Rocky Mountain uplift were a typical anticlinal uplift, the sandstones of 
the Cambrian period, which are the lowest beds of the conformable 'series 
exposed, would be found continuously along the eastern flanks of the Rocky 
Mountains wherever erosion had swept away the obscuring Tertiaries so 
that the edges of the folded rocks could be seen. 

Since it is evident, then, that the entire series of these beds could not at 
any time have arched over the present Archean exposures, the alternative 
presents itself that these exposures represent an ancient continent or island 
along whose shores they were deposited, a hypothesis which is borne out 
by the lithological character of the beds themselves, which bear abundant 
internal evidence, in ripple-marks, in prevailing coarseness of sediment, and 
in -the abundance of Archean pebbles in the coarser beds, that they are a 
shore-line deposit. The varying completeness in the series of sedimentary 
beds exposed at different points w.ould in this case be explained by unequal 
local erosion or elevation, by which the contact, now of a lower, now of a 
higher horizon, with the original Archean cliff would be laid bare. 

Inasmuch as the same evidence of shore-line conditions is found wher- 
ever the sedimentary beds adjoining the larger masses of Archean have 
been carefully studied, and as, moreover, in no part of the higher regions of 
these Archean ridges have relics of sedimentary beds been found, not even 
of the later Tertiary formations, as would be expected had they originally 
arched over these ridges, it is evident that these Archean islands have never 
been entirely submerged since they first appeared above the ocean level. 

The Colorado Range formed the most extensive of these ancient land- 
masses, and its outlines probably did not vary essentially from those of the 
present Archean areas. Extending from Pike's Peak northward to the bound- 
ary of the State, its dimensions were approximately one hundred and fifty 
miles in length by about thirty-five to forty miles in width. To the eastward 


it presented a continuous and regular shore line, broken only by a single 
narrow bay, separating the Pike's Peak mass from the mainland, and now 
known as Manitou Park. On the west, toward the parks, its original out- 
lines are as yet less certainly known, but though less regular they probably 
had a general parallelism with the eastern shore line. North and south this 
line of elevation was continued by a series of islands and submerged reefs 
to the Black Hills of Dakota on the one hand and into the present Ter- 
ritory of New Mexico on the other. 

The Parks. That the present valleys, known respectively as the North, 
Middle, and South Parks, have been more or less submerged in Paleozoic 
and Mesozoic and again in Tertiary times, and that at one time they formed 
a connected series of bays or arms of the sea, is proved by the sediments of 
those eras that are still found in them. Although the geology of the park 
region has not been studied in sufficient detail to afford complete data in 
regard to its past history, enough is known to furnish its general outlines. 

In some respects the present conditions of these depressions are those 
that prevailed in the earliest Paleozoic times; in others they have expe- 
rienced more or less change. Then as now the outlet or opening of the 
North Park was toward the north, of the Middle Park toward the west, and 
of the South Park toward the south. Oh the other hand, up to the close of 
the Cretaceous the North and Middle Parks were connected and formed a 
single depression; the present mountain barrier between the Middle and 
South Parks did not extend as far as their western boundaries, and a water 
connection existed between them, whose outlines cannot now be given 
exactly, owing to faulting and subsequent denudation; again, the waters of 
the South Park extended westward to the flanks of the land mass now form- 
ing the Sawatch Range. It seems probable that in earlier Paleozoic times 
only the North and South Parks were sufficiently submerged to receive the 
sediments that were washed down from the neighboring land masses, but that, 
as time went on, the waters became deeper or the sea bottom subsided, so 
that in Cretaceous times sediments were deposited continuously through the 
three valleys. In Tertiary times again, after they had been raised above 
the ocean-level, fresh-water lakes occupied the parks, and in their basins 


sedimentary beds were deposited, which have since been so extensively 
eroded off that the age or extent of these lakes cannot readily be determined. 

western uplift. The western boundary of the park area consisted of two 
or more distinct ridges or islands, forming, however, a general line of eleva- 
tion nearly parallel with that of the Colorado Range. These are the Park 
Range proper, on the west side of the North Park, and the Sa watch Range, 
now separated from the South Park by the Mosquito Range. Between these 
was the Archean mass of the Gore Mountains, which formed, with the 
southern extremity of the Park Range, the western wall of the Middle Park, 
of whose geological relations but little is definitely known. 

The present topographical boundary of the South Park on the west is 
the Mosquito Range, which has for this reason been also called the Park 
Range. Geologically, however, this name is less appropriate than topo- 
graphically, since prior to Cretaceous times no Mosquito Range existed, 
but the rocks which now form its crest still rested at the bottom of the sea. 
The Sawatch range forms the normal southern continuation of the Park 
Range as an original Archean land-mass; hence it seems advisable to avoid 
the use of the name Park Range in this latitude. 

The Archean land-mass of the Sawatch in Paleozoic times, judging from 
the almost continuous fringe of Cambrian beds encircling it, as shown on 
the Hayden maps, which may be assumed to represent a tolerable approxi- 
mation to its original outlines, was an elliptical-shaped area, trending a little 
west of north, with a length of about seventy-five miles and an extreme 
breadth of about twenty miles. Through the eastern portion of this area, 
and parallel with its longer axis, runs the valley of the Upper Arkansas 
River, now an important feature in the topography, but which during 
Paleozoic and Mesozoic times did not exist. 

The relative height of these mountain masses above the adjoining 
valleys must have been far greater then than now, since the sedimentary 
beds which surround them must have been formed out of the comminuted 
material abraded from their slopes. It is probable, however, that they 
were not the only land masses at that time, and future geological studies 
in this region will doubtless decipher many yet unopened pages in its 
past history. The great area of volcanic rocks to the southwest, whose 


culminating points are the San Juan Mountains, may very likely conceal 
the remains of a former land mass of equal, if not greater, dimensions than 
this. The present Archean areas to the south, in the Wet Mountain and 
Sangre de Cristo Ranges, may also, in part at least, have been land masses 
at those times. Moreover, the not infrequent occurrence of Cretaceous 
beds lying directly upon the Archean at points far away from any well- 
defined ancient shore line, suggest elevations and subsidences of which the 
geological studies thus far made in Colorado furnish no record. The areas 
already mentioned were, however, the most important elevations, since they 
are the only ones of which it may now be said with tolerable certainty that 
they have been permanent land surfaces through the long cycles that have 
elapsed since the commencement of the Paleozoic era. Their considera- 
tion, therefore, is all that is necessary for the purposes of the present study. 

Mountain structure. It is no longer assumed, as it was in the early days 
of geology, that the elevation of mountains is the result of a vertically 
acting force or a direct upthrust from below. On the contrary, the gen- 
erally received contraction theory, which is the one that best accords with 
all observed facts of geological structure, supposes that it is horizontally 
acting forces that have uplifted them. According to this theory, during 
the secular cooling of the earth from a molten mass, a solid crust was first 
formed on its exterior. As cooling and consequent contraction of the whole 
mass went on, this first-formed crust, in order to adapt itself to the reduced 
volume of its nucleus, also contracted ; but, as it was more or less rigid, this 
contraction resulted in the formation of wrinkles or ridges on its surface, 
which there is considerable evidence to show occupied essentially the same 
lines that the present mountain systems of the world do. Whatever the 
determining cause that originally fixed these lines, the earth's crust along 
them would have been compressed, plicated, and probably fractured, and, 
in subsequent dynamic movements resulting from continued contraction, 
they would have constituted lines of weakness along which the effects of 
these movements would have found most ready expression. 

Whether the consolidation of the entire earth- mass is already com- 
pleted, or whether there still remains a molten nucleus towards its centre, 
is a purely speculative question, upon which geologists are not yet in entire 


accord, and whose discussion would not be appropriate in a memoir like 
the present, which has to do with observed facts and with theories only 
so far as they are necessary for a proper comprehension of these facts. It 
is an observed fact that in the great mountain systems are found the most 
intense expression of the compression of the crust, in plications and in great 
faults. It is also an observed fact that along these lines of elevation and 
of consequent fracturing of the crust, have occurred the most extensive 
extrusions and intrusions of molten or eruptive rock, whatever may have 
been their source whether from a fluid center or from a fluid envelope 
between a solid center and a solidified crust, or from subterranean lakes 
of molten rock at different and varying points beneath the crust. It may 
likewise be considered a fact of observation that the tangential or horizontal 
thrust which the contraction theory requires most readily accounts for 
the plication arid faulting of the sedimentary beds which geological study 
discloses. This thrust may be best conceived as the expression of two 
forces of compression : a major force acting at right angles to the longi- 
tudinal axis of the mountain system, or east and west, and a minor force 
acting in a direction parallel with that axis, or north and south. 

The geological structure of the Rocky Mountains forms as marked a 
contrast to that of the regions adjoining it on either side as do its topo- 
graphical features. On the Great Plains, which stretch in an almost unbroken 
slope from their eastern base to the Mississippi River, or, it might be said, 
to the western foot of the Appalachians, the strata which form the surface lie 
in broad undulations, whose angles of dip are so gentle as to be scarcely 
perceptible to the eye, and which are apparently broken by no important 

In the Colorado Plateau region, which extends from their western edge 
to the base of the parallel line of uplift of the Wasatch, the beds seem as 
horizontal as when they were originally deposited, but along certain lines 
abrupt changes of level are brought about by sharp monoclinal folds, accom- 
panied by or passing into faults, and having great longitudinal extent. 

In the intervening mountain region the strata are compressed against 
the original land masses and flexed until the limit of tension is reached, 
when by great displacements, often measured by thousands of feet, their 


edges are pushed past and over each other, the movement of both folds and 
faults showing that the force which produced them was acting from either 
side toward the center of the original land masses. 

As contrasted with the Basin region west of the Wasatch uplift, the 
folds of the Rocky Mountains show a greater plasticity in the sedimentary 
strata by their relative sharpness, the anticlines and synclines in the former 
having more gentle and equal slopes, while in the latter they often have 
the form of an S, with one member almost bent under the other into an 

Compared with the remarkably compressed folds of the Appalachians, on 
the other hand, where the isocline may be considered the type structure, the 
flexures of the Rocky Mountains show that the sedimentary rocks are far from 
possessing the great plasticity and compressibility that they have intheformer. 
The contrast between the eastern and western mountain systems, in respect 
to the relative plasticity of their strata, is so marked that it would seem that 
the reason therefor must be readily apparent. It is not that the beds in the 
former are thinner ; on the contrary, the corresponding Paleozoic formations 
are many times thicker in the Appalachians than in the Rocky Mountains. 
It is to be* remarked, however, that in the former eruptive rocks are com 
paratively rare, especially those of Mesozoic and Tertiary age, while in the 
Rocky Mountains they are most abundant and in the western part of the 
Basin region they form the greater part of the surface ; to this fact may 
probably be ascribed, as will be shown later, the less plastic condition of 
the earth's crust in the latter regions. 

In the character of these eruptive rocks, again, there is a marked con- 
trast between the Rocky Mountains and the Basin region of Nevada. In 
the latter they almost exclusively belong to the Tertiary volcanics, approach- 
ing in character the lavas of modern volcanoes, the older and more crystal- 
line varieties, corresponding to the Mesozoic porphyries of Europe, having 
been rarely 'observed on the surface. In the Rocky Mountain region, on 
the other hand, while the Tertiary eruptive rocks are often developed on a 
very large scale, the earlier and more crystalline varieties seem to have an 
equal and even greater importance, if not in the actual amount of surface 


they occupy, certainly in the influence which they have had upon the con- 
centration of mineral formation. 

In that portion of the Rocky Mountain region under consideration there 
is a noticeable connection between the structural lines and those along which 
eruptive action has been most active. The latter correspond with the lines 
of weakness, of greatest folding and faulting. Leaving out of consideration 
the dikes which traverse the Archean rocks, which, though numerous, are 
of relatively small mass, the eastern uplift gives evidence of little eruptive 
activity, it being shown only by a few isolated outflows of Tertiary lavas. 
Along the line of the parks, on the other hand, both earlier and later erup- 
tions are so frequent that their outcrops form an almost continuous line from 
north to south parallel with the western uplift, while along the west base of 
the latter the Elk Mountains, the head of White River, and the Elk Head 
Mountains in Wyoming have apparently been the scenes of most violent and 
repeated eruptions during both Mesozoic and Tertiary times. 


Topography. That portion of the Mosquito Range the study of whose 
geological structure was considered necessary for a proper comprehension 
of the ore deposits of Leadville is shown in relief on Atlas Sheet V. It 
comprises a length of 19 miles along the crest of the range, and in width 
includes its foot-hills, bordering the Arkansas Valley on the west and South 
Park on the east, a slope in the one case of seven and one-half miles and 
in the other of about nine miles in a direct line. This is essentially an 
alpine region, scarcely a point within the area of the map being less than 
10,000 feet above sea level. 

In this area the range has a sharp single crest trending almost due 
north arid south, the Echelon structure being, however, developed on the 
northern and southern limits of the map respectively. To the west this 
crest presents abrupt escarpments, descending precipitously -into the great 
glacial amphitheaters which exist at the head of almost all the larger streams 
flowing from the range. The spurs have extremely irregular, jagged out- 
lines, resulting from the numerous minor hills which rise above the average 
slope. Within a few miles of the valley bottom, however, their form sud- 


denly changes, and from sharp serrated ridges they become broad, gently 
sloping mesas or table-lands. On the eastern side, though the descent into 
the glacial amphitheaters is almost as precipitous, the average slope is much 
less steep, and the spurs as a rule descend in long sweeping curves, widen- 
ing out gradually as they approach the valley. 

The spurs on either side of the range are thickly covered with a forest 
growth of alpine character, reaching from the valleys of the streams up to 
an average altitude of 11,700 feet, the upper limit varying somewhat with 
the more or less favorable conditions of the surface, and extending appar- 
ently somewhat higher on the western than on the eastern slopes. 

In the northern portion of this area, between the heads of the Arkan- 
sas and Platte Rivers, the main crest of the range, which has hitherto fol- 
lowed an almost straight line, takes a bend en echelon, and is continued on a 
line removed about two miles to the eastward, resuming, however, its orig- 
inal line just beyond the limits of the map. The massive formed by the 
three peaks, Mounts Cameron, Bross, and Lincoln, the last the highest point 
within the area mapped, lies still to the eastward of this crest and is topo- 
graphically an almost independent uplift. Sheep Mountain and the ridge 
which extends southeastward from it also form an apparently abnormal 
feature in the topography of the eastern slope. 

The sketch given in Plate III shows the general outlines of the eastern 
slopes of the Mosquito Range and the basin of the South Park, as seen 
from a western spur of Mount Silverheels. The sky-line of the western half 
is the crest of that portion of the range included in the map which lies south 
of Mosquito Peak, the low gap is that of Weston's pass, beyond which is 
the Buffalo Peaks group. The various gulches south of the Mount Lin- 
coln massive are indicated by name, and the lines of outcrop on their 
walls are somewhat strengthened to show the geological structure, which 
will be explained in detail in Chapter IV. Buffalo Peaks are 25 miles 
distant from the point of view, and the volcanic hill in the extreme left- 
hand corner of the sketch, seen across the South Park plain, is over 40 
miles distant. The little hill on the edge of the plain, and on a line with 
the eastern spur of Buffalo Peaks, which forms the continuation of the 
Sheep Mountain ridge, is Black Hill, which lies just beyond the extreme 


South Park 

Buffalo Peaks 

Julius Rien & Co. lith 



Sheep M . 

White Ridge. 

Gemini Peaks 

Dyer M* 

S.K Kmmon. Geolofiist-m- Charge 



southeast corner of the Mosquito map. The base of this hill is 10,000 feet 
above the level of the sea. 

It were scarcely possible to select an alpine region more admirably 
adapted to illustrate the interdependence of topographical and geological 
structure than that chosen for this study. The gentle slopes of the eastern 
spurs follow the inclination of the easterly dipping beds of Paleozoic rocks 
which form their surface, and which remain in broad sheets, like the covering 
of a roof, to protect the underlying Archean schists from erosion. Where 
they have been cut through, first by the erosive action of glaciers and 
later by the corrasive action of mountain streams, to their stratified structure 
is due the formation of the almost perpendicular cliffs which form the canon 
walls of their streams. The generally abrupt slope immediately west of 
the crest is due to a great fault extending along its foot, in virtue of whose 
movement the western continuation of the sedimentary beds, which slope 
up the eastern spurs and cap the crest itself, are found at a very much 
lower elevation on the western spurs ; while the jagged outline of the 
western spurs is due to a series of minor faults and folds, crossing them 
nearly at right angles. The secondary uplift of the Sheep Mountain ridge 
on the eastern slopes is the expression of a second great line of fault and 
flexure, whose direction, like that of the ridge itself, forms an acute angle 
with that of the main crest. The elevation of the Mount Lincoln massive 
is the result of a combination of the forces which have uplifted the Mosquitc 
Range and of those which have built up the transverse ridge which sepa- 
rates the South from the Middle Park. 

In the later topography of the range the results of the action of a 
system of enormous glaciers are seen in the immense amphitheaters which 
form the heads of its main streams, and in the characteristic V-shaped 
transverse outlines of the valleys descending from them. Finally, the mesa- 
like character of the lower end of the western spurs toward the Arkansas 
Valley is due to the existence beneath their surface of comparatively undis- 
turbed beds deposited at the bottom of a lake formed at the head of that 
valley by the melting of the ice at the close of the first portion of the 
Glacial period. 


The evidence furnished by the deposits of this lake affords an interest- 
ing confirmation of the deduction already made by geologists from the study 
of the glacial drift in Europe and in the Eastern States, arid by Messrs. 
King and Gilbert from their study of the lake deposits of the Basin regions 
of Utah and Nevada; namely, that the Glacial period presented two maxima 
of- cold, with an intervening warmer period during which the ice was 
partially melted and vegetation flourished. The general character of the 
stratified deposits of the Arkansas Lake shows that they must have been 
carried down during a time of great floods and that they are formed largely 
of rearranged moraine material. The thickness of these deposits proves the 
the existence during a long period of a lake which during part of the year 
was not frozen ; their position shows that the shores of the lake extended 
several miles to the eastward of the Arkansas Valley. Finally, the facts 
that these beds are deeply buried beneath surface accumulation of detrital 
material and that the moraines of now extinct glaciers extend out beyond 
the original shore-line of the lake and rest above its beds, prove that subse- 
quent to the draining of the lake another set of glaciers, formed during a 
later period of cold, covered the slopes of these mountains and carved out 
to a greater depth the present valleys. 

Geological history. Although now so prominent a feature in the topogra- 
phy of the Rocky Mountains, the Mosquito Range, from the sources of the 
Arkansas River to the southern end of the main Arkansas \ 7 alley, is geolog- 
ically a part of the Sawatch uplift. It was from the abrasion of the land 
surfaces exposed in the Archean island which occupied the present position 
of the Sawatch range that the sediments which constitute its stratified beds 
were doubtless in a great measure formed. In the seas that surrounded this 
island during Paleozoic and Mesozoic times was deposited a conformable 
and, as far as present evidence shows, an almost continuous series of coarse 
sandstones and conglomerates, alternating with dolomitic limestones and 
calcareous and argillaceous shales. The geology of the Rocky Mountains 
has not yet been studied in detail over a sufficiently extended area to afford 
data for tracing the history of the elevations and subsidences to which the 
region as a whole may have been subjected, or of the alternate recessions and 
advances of ocean waters during this long lapse of time. The examination 


of these beds made during the present investigation furnishes some evidence 
of a shallowing of these seas, and perhaps even of the existence of some 
land surfaces subjected to erosion during part of this time. Still, the absence 
of non-conformity in the successive strata deposited and their great uni- 
formity throughout the area studied show that no violent dynamic move- 
ment took place before the great disturbance at the close of the Cretaceous, 
which extended throughout the whole of the Rocky Mountain system and 
was doubtless the main factor in producing its present elevation. 

During this long period of conformable deposition there was an accu- 
mulation in this area of 10,000 to 12,000 feet of sedimentary beds. Toward 
the latter part of this period, possibly very near its close, there was an exhi- 
bition of intense eruptive activity, during which enormous masses of molten 
rock were intruded through the underlying Archean tioor into the overly- 
ing sedimentary deposits, crossing the beds to greater or less elevations 
and then spreading out in immense sheets along the planes of division 
between the different strata. It is not possible at present to define all the 
points at which these eruptive masses forced their way up, although they 
were doubtless very numerous and widely spread throughout the region; 
but the negative evidence obtained proves that the intrusive force must 
have been almost inconceivably great, since comparatively thin sheets of 
molten rock were forced continuously for distances of many miles between 
the sedimentary beds. That the eruptions were intermittent and continued 
during a considerable lapse of time is proved by the great variety of erup- 
tive rocks now found and by the fact that a given rock in one place pre- 
cedes and in another follows a second. It might naturally be thought that 
this .eruptive activity must have been coincident with or immediately sub- 
sequent to a great dynamic movement; but that it preceded the movement 
at the close of the Cretaceous, which caused the uplift of the Mosquito 
Range as well as of the other Rocky Mountain Ranges, is proved bj: the 
fact that these interbedded sheets of eruptive rocks, porphyries and porphy- 
rites, are found practically conformable with their bounding strata, and, like 
them, folded into sharp folds and cut off by faults. The intrusion between 
the strata of such vast masses of rock which in some cases reached a 
thickness of from 1,000 feet to 2,000 feet, and of which in other cases sue- 


cessive beds varying from 50 feet to 200 feet in thickness are now found 
intercalated between alternate strata to the number of 15 or 20 in a single 
section must necessarily have produced great irregularities in the once 
level surface of the then existing crust; but these irregularities were largely 
obliterated by the dynamic movements which followed, and the only traces 
still remaining are variations in the strike of the inclosing beds, which show 
a tendency to curve around any concentration of eruptive masses. 

At some time during the long period which intervened between the 
final deposition of the latest sediments of the Cretaceous epoch and the 
succeeding deposition of Tertiary strata, and during which the waters of 
the ocean gradually receded from the Rocky Mountain region, the pent-up 
energy of the force of contraction of the earth's crust, which had accumu- 
lated during ages of comparative geological tranquillity, found expression in 
intense and prolonged dynamic movements of the rocky strata forming the 
immediate crust of the earth in this region. These dynamic movements in 
their simplest form may be conceived as a pushing together from the east 
and from the west of the more recent stratified rocks against the relatively 
rigid mass of the already existing Archean land masses, and a consequent 
folding or crumpling of the beds in the vicinity of the shore-lines, where, 
owing to the break in the continuity of the strata and the more irregular 
character of the floor upon which they rested, the conditions were more 
favorable to the crumpling movement than they would be, for instance, in 
the open plains, where a great thickness of level and hitherto undisturbed 
beds offers no lines of weakness to favor a commencement of folding. It is 
here a question only of the movement of the distinctly stratified beds, 
because it is in these alone that the resulting flexures can be accurately 
studied and mapped out; but it is evident that the crystalline and already 
violently contorted beds which formed the Archean land masses must have 
also partaken in the resulting movements, and their axial regions have been 
lifted up to a great elevation, of which the present height of the culminating 
peaks of the Rocky Mountains, formed as they are in the majority of cases 
exclusively of Archean rocks, is only a very much modified expression. 
Contemporaneously with the east and west movements (the expression of 
the major force of contraction in this region), there acted also a minor force 


of contraction in a north and south direction, whose effects can now be seen 
along the eastern foot-hills in gentle lateral folds, their axes approximately 
at right angles to the trend of the range, and whose presence is indicated 
by a sudden bend or curve in the line of sedimentary outcrop, where at one 
point, owing to a local synclinal, the beds have been more or less preserved 
from erosion, and again where, owing to the crossing or coincidence of 
crests of the folds, like those of waves crossing each other, is found an 
otherwise unexplainable steepening in the dip of the strata. 

It must be borne in mind that, while this great dynamic movement is 
denned as occupying a certain lapse of geological time and its principal 
effects were brought about within that time, it is not to be regarded as a 
sudden convulsion, like that of an earthquake, though such disturbances 
may have occasionally occurred. On the contrary, it must be conceived 
to have been rather a slow and gradual movement, extending over a period 
of time of which human experience can form no adequate conception. 
Moreover, as will be shown in the detailed study of the region, it can be 
proved that in a modified degree this movement has been continued into so 
recent a period as that following the Glacial epoch, and may very probably 
be going on at the present day, although, owing to the great area involved, 
it has been impossible to obtain any demonstrable proof of its actual exist- 

Mineral deposition. It was during the period which intervened between the 
intrusion of the eruptive rocks and the dynamic movements which uplifted 
the Mosquito Range that the original deposition of metallic minerals in the 
Leadville region took place. These original deposits were probably in the 
form of metallic sulphides, though as now found they are largely oxidized 
compounds, and therefore the result of a secondary chemical action; although 
during this secondary action they may have been to a slight degree removed 
from their original position, their relation as a whole to the inclosing rocks 
must remain essentially the same. Their manner of occurrence and the 
probability that they were derived, in great part at least, from the eruptive 
rocks themselves prove that they must be of later formatioq than the latter, 
while the fact that they have been folded and faulted together with the 
inclosing rocks, both eruptive and sedimentary, shows that they must have 



been formed prior to the dynamic movements, and that they are therefore 
older than the Mosquito Range itself. These deposits were formed by the 
action of percolating waters, which, having taken up certain ore materials 
in their passage through neighboring rocks, deposited them in a more con- 
centrated form in their present position. This process may have taken place 
while the sedimentary beds were still covered by the waters of the ocean, 
and the waters therefore have been derived from it; whether this was actu- 
ally the case or not cannot be known until the age of the eruptive rocks is 
more exactly determined. However, as it is already known by the estua- 
rine character of its fauna that the latest Cretaceous formation must have 
been deposited in an already shallowing ocean, it seems probable that the 
area occupied by the Mosquito Range may have already emerged from the 
ocean at this time. 

Structural results of the dynamic movements. Before proceeding to a detailed 

geological description of the region included in the Mosquito map (Atlas 
Sheets VI and VII), which represents the results of the dynamic movements 
and of subsequent erosion, it may be well to give a brief summary thereof, 
thus reversing the natural order, for the benefit of those readers who may 
not have time or inclination to follow all the details of Chapter IV. 

The average or major strike of the sedimentary beds and of the axes of 
the principal folds is northwest magnetic, or N. 30 W., but in some cases a 
strike due north and south is observed. In these two directions are seen 
the influence of the shore lines of the Sawatch island, against which the 
sedimentary strata were compressed; for, while this area lies mainly along 
the eastern shore line which has a north and south direction, in the north- 
ern part the beds had already commenced to sweep round to the westward 
along the northern shore line of the island. To the south of this area the 
crest of the Mosquito Range itself marks the eastern limit of Paleozoic 
beds, while from South Peak, near Weston's pass, northward this limit bends 
to the northwest toward the mouth of the east fork of the Arkansas. 
Beyond this line to the west everything is Archean; to the east of it Archean 
exposures are found only where denudation has removed their previous cov- 
ering of Paleozoic and later beds; it may be assumed, therefore, to repre- 
sent approximately the original shore line of the Paleozoic ocean. 


The uplift of the Mosquito Range was not the simple pushing up of the 
beds into a monoclinal fold, as might appear at first glance from the seem- 
ingly regular dip of the beds from the crest down it" eastern slopes, but a 
somewhat irregular plication of them into anticlinal and synclinal folds, and 
their fracturing by faults, which have the same general direction as the axes 
of the 'folds without coinciding exactly with them, and which often pass 
into folds at their extremities. The anticlinal folds have as a rule a very 
steep inclination, sometimes nearly vertical, on the west side of the axis 
and a more gentle slope to the east, thus approaching the form of the 
isocline. It is along this steeper slope that the fracturing has generally 
taken place, and the fault may thus follow the axis of a syncline or of an 
anticline, according as it runs to the one side or the other of this steep slope. 

The north and south direction of the main crest of the range is evi- 
dently determined by the great Mosquito fault, which, starting at some aa 
yet unknown distance beyond the northern boundary of the map, follows 
the foot of the steep slope west of the crest to the region of the Leadville 
map, where for a short distance it bends somewhat further to the westward 
and is thence continued southward in the Weston fault, which passes into a 
synclinal fold south of Weston's pass. 

From the Mosquito fault just north of Mosquito Peak branches off the 
next most important fracture plane, the London fault, which runs in a south- 
easterly direction across the eastern spurs of the range. The line of this 
fault passes just east of the axis of a most pronounced anticlinal fold across. 
London Mountain and Pennsylvania hill to Sheep Mountain, on the side* 
of which the folding can be most distinctly traced along the canon walls. 
To the south of Sheep Mountain it apparently coincides with the axis of 
the anticlinal fold which forms Sheep ridge, and with it gradually dies out 
and passes under the level plain of the South Park. 

The geological structure of the Mosquito Range is simplest toward the 
south and becomes more complicated as one goes north, reaching the ex- 
treme of complexity opposite Leadville. Near Buffalo Peaks, a few miles 
beyond the southern limits of the map, it seems to be a simple monoclinal 
fold, the western slopes being entirely of Archean granite, and the crest 


formed by Cambrian quartzites dipping gently eastward and resting uncon- 
formably on the Archean. 

At the southern edge of the map an anticlinal and synclinal fold comes 
in to the east of the monocline. Here the range has a double crest en e"che- 
lon, divided by the longitudinal valley of Weston's pass, which runs north- 
west magnetic following the direction of the strike. The ridge of South Peak 
to the west of the pass is formed by a monocline of easterly-dipping Cam- 
brian and Silurian beds. The valley of the pass itself is formed by a com- 
pressed synclinal fold in Carboniferous strata, along the eastern side of 
which runs the Weston fault, bringing up the Archean and Cambrian on 
its east side. The ridge bounding the valley on the east, which is the south- 
ern end of the main crest of the Mosquito Range, is an eroded anticlinal 
fold, from whose crest the overlying Paleozoic strata have been almost en- 
tirely removed, leaving the core of Archean exposed. On the very sum- 
mit of Weston's Peak a small patch of Cambrian quartzites is left, a remnant 
of the crest of this fold, and at its western base the same beds are found in 
a vertical position adjoining the fault, while on the more gentle slopes of 
the eastern spurs are found the regular succession of easterly-dipping Pale- 
ozoic beds belonging to the eastern member of the anticline. The ridge 
sinks to the southward, and over its southern end the arch of Paleozoic beds 
is still left entire, but the anticlinal fold also sinks to the southward and 
entirely disappears beyond the limits of the map. 

The same general structure continues northward as far as Empire Hill, 
but a short distance from the southern edge of the map a second anticlinal 
fold, that of Sheep Ridge, comes in at the extremity of the eastern slope of 
the range, while from its steep western slope erosion has removed all trace 
of the synclinal fold seen on Weston's pass, leaving only the easterly-dip- 
ping Paleozoic beds belonging to the monocline on the west of the fault, 
and the Archean on its east side ; the crest of the range is formed of east- 
erly-dipping Paleozoic beds, or, where these have been eroded away, by 
Archean schists and granite. This double anticlinal structure is best shown 
in Section Gr (Atlas Sheet IX), which is drawn at right angles to the strike, 
and in which the supposed form of the eroded synclinal is shown by dotted 
lines. The line of this section also crosses two secondary anticlinals or 


minor waves in the strata, which are the almost invariable accompaniments 
of the larger folds. 

In this southern area the older eruptive rocks are but little developed, 
their only representative being a thin but persistent sheet of White Por- 
phyry above the Blue limestone. This increases in thickness from about 
fifty feet at Weston's pass to over a thousand feet at its supposed source in 
White Ridge, on the north side of Horseshoe gulch. 

In the middle region of the area mapped, through an east and west 
zone which includes the principal mines of Leadville and vicinity, the de- 
velopment of bodies of earlier eruptive rocks is so great that the structure 
of the sedimentary beds is obscured and not always easy to trace. On the 
eastern slopes the double anticlinal structure continues as far north as Mos- 
quito Peak, at the head of Mosquito gulch. The great Sheep Mountain fold, 
with the London fault cutting through its steeper western side, gradually 
converges toward the crest of the range. Views of the sections of this 
fault-fold afforded by the canons of Horseshoe and Big Sacramento gulches 
are seen in Plates XV, XVI, and XVIII. East of this fold the strata 
slope gently eastward, with a slight secondary fold traceable along the 
extreme foot-hills. Between the Sheep Mountain fold and the crest of the 
range the strata of the gradually narrowing syncline are cut across by 
the two great eruptive bodies of White Porphyry and of Sacramento Por- 
phyry, in White Ridge and Gemini Peaks, respectively, which are accom- 
panied by a slight displacement. The nearly horizontal Paleozoic beds 
forming the crest and eastern member of the main anticline extend some- 
what to the west of the topographical summit of the range, but the western 
member of the anticline and the succeeding syncline (if it extended so far 
north) are either removed by erosion or buried beneath sheets of porphyry^ 

On the western slopes in this zone the sedimentary strata, now greatly 
augmented in thickness by interstratified sheets of porphyry and extend- 
ing nearly to the valley of the Arkansas, are flexed into a number of minor 
folds and broken by many shorter faults, most of which pass at either end 
into anticlinal or synclinal folds. This is the area which is included in the 
detail map of Leadville and vicinity and which is described at length in 
Chapter V. It is traversed by seventeen larger and smaller faults and has 


many anticlinals and synclinals, in which the prevailing dip of the beds is 
to the eastward and the throw of the faults is mainly an uplift to the east. 

The area west of the Mosquito fault and north of the Leadville region 
is mainly occupied by beds of the middle member of the Carboniferous and 
by porphyry sheets, flexed into gentle folds of varying directions, but appar- 
ently not broken by faults. This region is already at some distance from 
the ancient shore line, which is marked by the outcrops of Cambrian and 
Silurian beds. These bend to the westward around the head of Tennessee 
Park, and reach well up on the north slopes of the Sawatch in the Eagle 
River region; but, while the sedimentary beds bend thus in general strike 
to the westward, the Mosquito fault and the crest of the range which has 
been uplifted by its movement continue on unchanged in their trend. 

North of Mosquito Peak is a large area in the higher part of the range, 
including the splendid amphitheaters in which the Platte and Arkansas 
Rivers rise, where the overlying Paleozoic beds have been entirely removed 
and only Archean exposures, traversed by dikes of earlier eruptive rocks, 
now remain. 

East of this area the flanks of Loveland hill and the massive of Mounts 
Bross and Lincoln are occupied by easterly dipping Paleozoic beds, which 
evidently are the eastern member of a broad anticlinal fold ; but of the 
actual structure of the beds which once arched over the Archean area 
there is nothing left to tell. It is probable that there were folds here simi- 
lar and more or less parallel to the Sheep Mountain fold, as has been indi- 
cated in a general way by the dotted lines in the sections which cross this 
region. A partial proof of this is afforded by a deep synclinal adjoining Mos- 
quito fault on the west, somewhat similar to that on Weston's pass, which 
is found at the base of Bartlett Mountain, at the northern edge of the map; 
its axis has more westerly direction than the plane of the fault. In this 
northern area there is also a great development of earlier eruptive rocks 
as contrasted with the southern half of the region, though they have less 
relative importance than in the middle zone, which includes the immediate 
vicinity of Leadville. The greater proportion of these bodies are in the 
form of intrusive sheets, interstratified with Paleozoic beds; but the dike 
form is also found, more especially in the Archean exposures in the ampin- 


theaters along either side of the crest of the range. These dikes are some- 
times observed cutting up through the Archean into the overlying sedi- 
mentary beds and then spreading out in sheets between the strata. 

Displacement. The movement of displacement of the faults throughout 
this area has been, with a few unimportant exceptions, an upthrow to the 
east. The maximum movement of any one fault is that of the Mosquito 
fault, at the northern edge of the map, which is about five thousand feet. 
In general the movement of the individual faults decreases to the south- 
ward until they gradually pass into folds and it becomes nil. The aggre- 
gate amount of displacement, however, summed up along east and west 
sections, increases toward the middle of the region, where the development 
of sheets of eruptive rocks is greatest, and decreases as these become less 
important; thus, as above mentioned, the displacement at the northern edge 
of the map is about five thousand feet. In the middle region, where the 
faults are numerous, the aggregate displacement is 8,000 to 10,000 feet, 
and across Sheep Mountain and Weston Peak it has decreased to 3,500 feet, 
becoming nothing at all just beyond the southern limits of the map. 

volcanic rocks Thus far only the earlier eruptive rocks have been men- 
tioned, for the reason that they alone were involved in the folding and 
faulting. Later eruptions of Tertiary volcanic rocks have taken place since 
the folding, and probably after erosion had done the greater part of its work 
in the removal of Paleozoic sediments. These eruptions within the area 
of the map consisted of rhyolitic lavas, of which the two most prominent 
outpourings were at the extremities of this area, the one forming the mass 
of Chalk Mountain north of the east fork of the Arkansas and some smaller 
bodies to the east of Fremont's pass, the other that of Black Hill, on the 
extreme southeastern edge of the area in South Park. Besides these there 
are small bodies in the granite and in the Cambrian quartzite at the west foot 
of Empire Hill. A few miles south of the southern limit of the map -is an 
important volcanic eruption of andesitic lava, cutting across both the Archean 
and the Paleozoic beds, which forms the high mass of Buffalo Peaks. These 
later eruptions, however, so far as can be determined, had no influence upon 
the ore deposits of the region. 


General erosion. It is now impossible to determine how much of the ero- 
sion that has removed the crests of these folds and denuded such large 
masses of Archean rocks was accomplished earlier than the Glacial period, 
but it is evident that the carving and shaping out of the valleys which score 
the flanks of the range has been mainly accomplished since that time. It 
is, moreover, not absolutely certain that this area was entirely covered by 
later beds than the Triassic, since the only proofs that Jurassic and Cre- 
taceous strata also extended over it conformably are founded on the fact that 
no unconformability between Jura and Trias has yet been observed here. 
On the other hand, no opportunity was offered for a detailed study of the 
relations of these two formations. That the beds of the Trias formed part 
of the conformable series and were deposited along the shores of the Sa- 
watch island is definitely proved, although they are no longer found within 
the area of the map, by the fact that just beyond its limits to the north and 
east, in the Ten-Mile and Mount Silverheels districts, respectively, they form 
a continuous and conformable series with the Carboniferous beds, are folded 
and faulted with them, and carry the same intrusive sheets of eruptive rocks. 

Arkansas Valley erosion. The manner and date of formation of the main 
Arkansas Valley is a matter of interesting speculation. It is evident, as 
has already been said, that it did not exist before the dynamic movements, 
which uplifted the Mosquito Range, and yet it must have already been a 
deep valley at the commencement of the Glacial period, since a large lake 
was formed in it during the first melting of the ice of that period, in whose 
bottom at least three hundred feet of sediments were deposited. Although 
no beds of undoubted Tertiary age have yet been recognized in it which 
would afford a definite date to reckon from, it is probable from structural 
evidence that a line of depression was formed by the elevation of the Mos- 
quito Range and the accompanying faulting, which corresponded approx- 
imately with the present general direction of the valley. A new drain- 
age system having thus been formed, the erosive agencies which have 
carried away so many thousand feet of rocks from the range itself gradu- 
ally deepened and enlarged this new depression, until it has now assumed 
those majestic proportions that make it a topographical feature of scarcely 


inferior importance to the great parks themselves, which date back to pre- 
Cambrian time. 

Glacial erosion. The detrital materials brought down from the adjoining 
mountains and deposited along the Arkansas Valley during the Glacial 
period show that the general form of the latter had already been determined 
before that time. These deposits, though of similar origin and lithologcal 
character, belong to two distinctly marked epochs. Those of the former 
constitute the so-called Lake beds, formed of detrital and mainly morainal 
material, brought down from the mountains by the freshets which occurred 
during the melting of the ice at the close of the first cold epoch of the 
Glacial period, and which formed stratified deposits at the bottom of the 
great lake at the head of the Arkansas Valley, which will be called the 
Arkansas Lake. These beds, which reached a thickness of at least three 
hundred feet, are now found on either side of the alluvial bottom of the 
present stream, forming the base of the mesa-like terminations of the mount- 
ain slopes and in some cases extending to an elevation of 1,000 feet above 
the present valley bottom, a height to which the angle of the deposition of* 
the beds could hardly have carried them and which gives evidence that the 
elevation of the range has continued in a modified degree since Glacial 
times. After the draining of this lake, in some manner not now to be 
traced, a second epoch of glacier formation set in, during which the new 
glaciers occupied the same positions as the older ones and continued the 
work of grinding and valley carving. They extended out over the Lake 
beds deposited during the warmer period, as proved by the present position 
of the lateral moraines of Iowa and Evans gulches. An immense amount 
of detrital material must have been accumulated on the slopes of the range 
by this second system of glaciers, and during the floods and freshets that 
must have accompanied their melting and recession this material was par- 
tially rearranged and spread out over the lower part of the Leadville region, 
both above the already existing Lake beds and in some cases over rock sur- 
faces not previously covered by these deposits. This rearranged moraine 
material has received the local name of "Wash." From the present regular 
and even surface of the lower spurs, where the Wash lies conformably over 
the Lake beds, it is evident that the former, like the latter, must have been 


deposited in quiet waters, like those of a lake rather than of a mountain 
torrent, for which reason it seems probable that a second Lake Arkansas was 
formed at the very close of the Glacial period, perhaps by the damming up 
of the valley by a terminal moraine, which in its turn finally broke its bar- 
riers and was drained of its waters, leaving- a basin-shaped valley in whose 
original bottom, as represented by the mesa-like spurs, the lower part of 
the present stream beds have been cut out. 

stream erosion. The valleys of the minor streams which head in the 
amphitheaters at the summit of the range were shaped out and took their 
general direction during the Glacial period. It is evident that their upper 
portions, at the heads of these amphitheaters, have been but little changed 
by later erosion, since glacial striae are still found in some cases on their 
present bottoms. 

The amount of erosion produced by rain and running water increases 
in direct ratio with the distance from the crest of the range. In the valleys 
of some of the larger streams running down from its summit this erosion 
*has cut to a depth of 500 feet below the valley bottom left when the gla- 
ciers receded, and many minor valleys, like California gulch, which do not 
head at the actual summit, have been entirely carved out by these agencies 
since the close of the Glacial period. 

Valleys. The valleys of the minor streams, or gulches as they are gen- 
erally called, may be divided in the vicinity of Leadville into three classes, 
according to age and manner of formation. They may be distinguished as 
(1) glacial valleys, (2) valleys of erosion, and (3) surface valleys. 

The first and oldest, which owe their main outline to the carving of 
glaciers, have in cro?s section a U-shape and head in glacial amphitheaters, 
from which they pursue a relatively straight course down the mountain 
slope. Their original form is more or less modified by subsequent erosion. 
To this class belong the larger valleys, often forming canons on the east 
side of the range, and the east fork of the Arkansas and Evans, Iowa, and 
Empire gulches on the west side. 

The valleys of the second class, which have been cut out of solid rock 
exclusively by the action of running water, have a V-shaped outline in 
cross section and a winding course, their direction being dependent on the 


unequal resistance offered by the peculiar position or texture of the rocks out 
of which they are carved. They also want the amphitheater-shaped head 
which characterizes the first class. They are more recent than the glacial 
valleys and have sometimes been cut out of their bottoms. The most 
striking example of this kind of valley is California gulch. 

The third class, which are of the most recent formation, are likewise 
valleys of erosion ; but they have been cut, not out of solid rock, but out of 
recent surface accumulations like the Lake beds, which have not yet be- 
come solid rock. They are relatively broad and shallow and are often dry 
for a great part of the year. They are like the shallow ravines and river 
valleys of the Great Plains and of the Nevada valleys, and like them proba- 
bly mainly carved by sudden freshets. Little Evans, Georgia, and Thomp- 
son's gulches are valleys of this class. On the map of Leadville and vicinity 
it will be seen that the geological outlines cross these valleys without the 
re-entering angle which they have on the lines of the, other valleys. 

Little Evans Valley drains the amphitheater on the south face of Pros 
pect Mountain, being separated from Big Evans Valley only by a moraine 
ridge formed by the glacier of the second epoch. It is thus proved that the 
amphitheaters were carved out by the earlier set of glaciers, since the glacier 
from the Prospect Mountain amphitheater was originally a branch of the main 
glacier from the Evans amphitheater, and it was the moraine of the second 
Evans glacier which, being placed across the mouth of the Prospect Mount- 
ain amphitheater, necessitated its seeking a new outlet for its waters. That 
at one time ice must have filled the amphitheaters to their brim, and been 
in places over 2,000 feet thick, is proved by their configuration and by the 
position of erratic blocks. 

In the region shown on the accompanying maps, the two main glaciers 
of the second epoch were the Evans and the Iowa. The latter had three 
heads, but its lower portion, as shown by the lateral moraines which remain 
on the sides of the present gulch, was straight and narrow. The later 
Evans glacier, however, spread out as it descended, having left a prominent 
moraine ridge along the north bank of the present stream at the foot of 
Prospect Mountain, while on the south side a somewhat disconnected moraine 
ridge follows approximately the course of Stray Horse gulch, the moraine 


material remaining being 250 feet or more thick in the Rothschild and Den- 
ver City shafts. The steep north face of Breece Hill below the present 
grade formed its southern wall, and below this it probably covered more or 
less completely all the region north of Stray Horse gulch, so that to its 
action is probably due the exposure of the valuable ore deposits of Fryer 
Hill, and also the removal of a great portion of them. 





The Archean rocks as developed in this district belong apparently to 
the very oldest of the crystalline sedimentary rocks, and on this ground may 
be considered as corresponding with the eastern Laurentian. As yet no 
systematic study of the Archean formations in the Rocky Mountain region 
has been made in accordance with which the different developments of 
Archean rocks may be classified as regards their age and correspondence 
with the different divisions made by eastern geologists. In the reports of 
the Survey of the Fortieth Parallel recognition was taken of the fact that at 
least two distinct developments of crystalline sedimentary rocks are found 
in the Rocky Mountain region. 

Of these, the one, consisting essentially of granites, mica and horn- 
blende gneisses, and amphibolites, being evidently the older, was considered 
to correspond with the Laurentian series; certain accessory occurrences of 
norite and beds of ilmenite and magnetic iron further allied it to this for- 

The second class, which was supposed to correspond to the Huronian, 
was found in rather limited development at Red Creek, near the Uinta 
Mountains, and along the Wasatch Range, and consisted of mica schists and 
quartzites, the former passing into paragonite schists similar to those of the 
St Gothard, with chloride and hornblendic rocks, in general of a less 
perfectly crystalline structure than the former. The Archean rocks of the 
Black Hills, which consist of a great variety of slates, phyllites, quartzites, 



and amphibolitic schists of singular composition, are also closely allied by 
their mineralogical character to this latter group. 

To the former of these classes belong the mass of the Archean rocks 
so largely developed throughout the whole Colorado Range of the Rocky 
Mountains, of which excellent sections are afforded by all the streams which 
flow out upon the Great Plains. Here in a very general way they seem to 
consist principally of gneisses resting on a central core of red, friable, 
coarse-grained granite. 

Although no opportunity has been had of making a study of the other 
Archean bodies of the Rocky Mountains, it would seem, from what has 
been seen in traveling across them, that the Archean of the Mosquito Range 
is distinguished from that of the Colorado Range by a greater prevalence 
of granite over schists, and in a very general way that the more schistoid 
rocks of the Mosquito Range are resting upon the almost entirely granitic 
mass of the Sawatch, which should therefore be considered the older. 

As shown in the section afforded by the cafions of the Mosquito Range, 
and hence in comparative nearness to the overlying sedimentary rocks, 
the Archean formation consists essentially of granite, gneiss, and amphib- 
olite. The granites are in many cases undoubtedly metamorphic and form 
bedded masses. In other cases there seems little doubt that they are erup- 
tive, but probably of Archean age, since they have not been found to intrude, 
into or contain fragments of the Paleozoic rocks. In the majority of cases 
the structural evidence was not decisive either way, but the texture, of the 
granite was decidedly that which is found characteristic of the metaphor- 
phi c types. 


The granites are prevailingly very coarse-grained, especially those in 
which evidences of bedding are found. If the classification given by Rosen- 
busch 1 be here adopted the greater part will belong to his class of granite 
in the narrower sense of the word, or granite proper, consisting, namely, of 
quartz, two feldspars, biotite, and muscovite. These granites always con- 
tain muscovite and variable biotite, but rarely if ever hornblende; where 

1 Mik. Physiog. tier mass. Gesteino. H. Rosenbnsch. Stuttgart, 1877, p. 18. 


biotite is absent it is due to a later alteration of the rock. In color they 
are gray or very frequently of a reddish tinge. The red color is sometimes 
very marked, and certain varieties are fully as fine in color as the famous 
Aberdeen granites. As an exceptional color is also found a reddish-yellow, 
due apparently to hydrated oxides of iron. 

Those which it has been thought might be of eruptive origin are gen- 
erally fine-grained, of gray color, and contain an abundance of biotite, 
whereas those which are distinctly metamorphic are generally coarse- 
grained, often red in color, and have a porphyritic structure owing to the 
prevalence of large twin crystals of orthoclase. Surfaces of the latter type 
often show such parallelism and rectangularity in the disposition of the long 
narrow prisms of orthoclase as to present a superficial resemblance to the 
so-called graphic granites. These coarse-grained metamorphic granites, 
especially when found in the immediate vicinity of the overlying sediment- 
aries, have sometimes a foliated structure approaching that of gneiss, but 
the direct passage of granite beds into gneisses was not observed. As typ- 
ical granites of the former or eruptive class, may be mentioned that found 
in the Platte Valley, north of Mount Lincoln; in Democrat Mountain, at the 
head of Buckskin gulch; and along the western slope of the main crest, 
opposite the head of Mosquito gulch. 

Of the second class typical forms are found at Bartlett Mountain and 
along the Arkansas Valley, which are distinguished from the former by the 
development of orthoclase in tabular twins, following the Carlsbad law, 
porphyritically distributed throughout the rock. That found at Leadville, 
generally in large erratic bowlders, and which has been considerably used 
as foundation stone, is a remarkably beautiful rock, the orthoclase having 
a delicate flesh-red tinge, while the groundmass, if such it may be termed, 
is a bright, clear-gray mass, rich in dark mica. 

The finer-grained granites of a deep blood-red color were observed on 
the ridge between Empire and Weston gulches, and also in the valley of 
Eagle River, opposite Tennessee pass. Yellow granite was found also in 
the last-mentioned locality and on the summit of Weston's pass. 

In addition to the above are masses of secondary origin, which occur 
in the form of huge white veins of extremely irregular outline, to which, in 


accordance with the custom now prevalent among German geologists, the 
term pegmatite has been given. These pegmatites consist of large inter- 
grown crystals of white orthoclase, microcline, and quartz, with irregular 
masses of muscovite, and are evidently of later formation, probably the 
filling in, by secretion from the surrounding rocks, of fissures and irregular 
openings formed in the mass by contraction or dynamic movement. 

Microscopic constitution. Besides the normal components, which are easily 
detected macroscopically viz, quartz, orthoclase and plagioclase feldspars, 
potash and magnesia micas the only constituent of importance revealed 
by the microscope is niicrocline, which occurs in all rocks examined except 
those of the type from Democrat Mountain This is often quite abundant, 
and seems to have been the last feldspar formed, which may be the reason 
for its superior freshness and freedom from particles of limonite and hema- 
tite, the abundance of which in the other feldspars causes their reddish 
color. The quartz grains are often full of fluid inclusions and hair-like 
needles. A few of the fluid inclusions were observed to be double, the 
inner substance being probably carbonic acid. 


The gneisses, which are next in importance to the granites, are more 
generally micaceous than those of the Archean along the Fortieth Parallel, 
among which the distinctly hornblende gneisses were the more prevalent 
They are much contorted and seldom exhibit very distinct bedding over 
large areas. In structure they present a great variety of forms, prevailingly 
the typical gneiss structure with fine, even grain and constant composition 
in the different layers, aside from the flat lenses of quartz or feldspar which 
are inserted between them. At other times a banded appearance is pro- 
duced by the alternation of layers in which biotite or hornblende prevail 
over quartz and feldspar. A porphyroidal structure is very marked in a 
variety from the South Platte amphitheater, caused by the development of 
large white orthoclase crystals, usually Carlsbad twins, reaching two to three 
inches in length, in a matrix of ordinary gneiss. The tendency to a granitic 
structure is locally noticeable, especially in the Twelve-Mile amphitheater. 
In composition the gneisses are prevailingly micaceous, hornblende being 


seldom present in large quantity, except in those rocks which are classed 
distinctly as amphibolites. Biotite is in some cases the sole mica, but 
frequently muscovite is associated with it in subordinate quantity. A careful 
search with the lens is often necessary to determine the presence of plagio- 
clase. The feldspars are generally white, but in the Mosquito, Horseshoe, 
and Twelve-Mile amphitheaters a pink or reddish color predominates. In 
these cases the pegmatite which forms veins in the schists is also pinkish. 

Microscopic constitution. A microscopical examination reveals the presence 
of microcline in small quantities, while ordinary plagioclase is very abundant, 
as is also muscovite frequently intergrown with the feldspars Apatite and 
ilmenite are the most common accessory minerals, the latter giving rise 
to titanite in the form originally called titanomorphite by von Lasaulx. 1 
Isolated rounded grains, which are nearly or quite colorless and but very 
faintly dichroic, are doubtless referable in part to titanite and in part to 
pyroxene of a variety near sahlite. The dark portion of a banded gneiss 
from the Arkansas amphitheater consists principally of quartz, three feld- 
spars, biotite. and hornblende. The last two minerals are often intergrown 
in a peculiar manner, the biotite leaves being parallel to the orthopinacoid 
of the hornblende. Ilmenite is abundant and passes by alteration into 
" leucoxene," which appears dull white by reflected light. This again passes 
into a granular mineral resembling titanite, although not very strongly 
dichroic. Blood-red films of hematite are discovered in the leaves of biotite. 

In the porphyritic gneiss of the Platte amphitheater microcline is an 
important element. One large grain of it contains inclusions of quartz and 
mica in considerable quantity. Muscovite, which is not prominent macro- 
scopically, is abundant in delicate plates intergrown with the feldspars, 
either parallel to the common crystal faces or without regularity. This 
muscovite seems to be original and not a decomposition product. An 
intergrowth of biotite and muscovite, whereby a crystal of the former is 
surrounded by a zone of the latter having the same orientation, was also 
observed. Quartz grains contain biotite crystals, needles of riitile (?), and 
double fluid inclusions with carbonic acid. The pink feldspar in gneiss 

'A. von Lasaulx, Nenes Jahrbnch fUr Min., etc., 1879, p. 568. 


from the Twelve -Mile amphitheater presents a confused intergrowth of 
different feldspars, a part being undoubtedly microcline. 



The amphibolites are the next in importance to the gneisses among 
the crystalline schists, and occur interstratified with them in layers of 
varying thickness, and sometimes in large lenticular bodies. Under the 
name amphibolite are here understood rocks of comparatively coarse grain, 
with less marked schistose structure than is common in hornblende schists 
proper, and also differing from these in that other minerals, particularly 
feldspar and quartz, occupy prominent positions beside the hornblende. 
They are of frequent occurrence throughout the Archean formation of this 
district and have a comparatively uniform structure, although sometimes 
showing a mottled appearance, from the concentration of hornblende in 
patches. Biotite and magnetite are often quite prominent in them. Pyrite 
is frequently visible macroscopically. 

Microscopic constitution. The microscope shows that orthoclase and pla- 
gioclase are present in about equal quantities, but that microcline, which 
was found in many gneisses, does not appear in the associated amphibolites. 
Hornblende occurs in stout, irregular individuals, and often contains inclu- 
sions of a clear, colorless mineral in minute rounded particles, which are 
probably quartz, although too small for certain determination. Amphibolite 
from Weston's pass contains hornblende which is so full of black ore-grains 
as to be opaque in certain cases. A fine striation parallel to the plane PW l 
was observed on the same hornblende. Apatite in its usual form is common 
to all. Titanite, as formed through the alteration of a titanium mineral, 
probably nigrine or rutile containing titanic iron, 2 is present in two cases 
in most typical form. The rutile has a dull-reddish hue by reflected light 
and is surrounded by titanite in clear oval grains. Two occurrences, viz, 
from Buckskin gulch and from Twelve-Mile amphitheater, show the mode 
of formation of titanite with exceptional clearness. 

'C. W. Cross, Smdieu uber bretoniscbe Gesteiiie; Min. uml petro. Mitth. Von G. Tscheroiak. 
Nene Folge, III., p. 386. 

Rammelsberg, Mineralchemie, Her Theil, 2te Aullago, p. 169. 


These two rocks, gneiss and amphibolite, constitute the main mass of 
the Archean schists, mica schists, phyllite, and other thinly bedded rocks not 
occurring in any well defined bodies. Peculiar schistose forms do appear 
in the gneissic series, but are subordinate in every respect, with only local 
extension, and of abnormal constitution. In the contorted state of the strata, 
the tracing out of the relations of these bodies to the gneiss, while extremely 
interesting, would have taken much more time than could have been devoted 
to this subject. A few examples will show the interesting nature of these 

On the north face of Mount Lincoln occurs a contorted schist of dark 
color, in which the naked eye can determine biotite and small flakes of 
glistening muscovite. The microscope shows that the two micas form 
nearly the whole rock, the compact appearance being due to extremely 
minute flakes of biotite, often so small as to require a power of 800 diame- 
ters to distinguish them clearly. Between these two elements, in varying 
quantity, is a mass appearing between crossed nicols like the decomposition 
product of orthoclase in many of the older rocks, where muscovite in tiny 
flakes has been the chief mineral formed ; this substance is here very uni- 
form in composition, giving the brilliant polarization colors of such an 
aggregate, and, as no feldspathic substances can be detected, it remains 
uncertain whether this muscovite comes from orthoclase or is original, cor- 
responding to the minute leaflets of biotite. No hornblende is visible. 
Tourmaline in bundles and brushes is the next most abundant element, 
being brown in ordinary light, with a tinge of red or blue; a few small 
grains of quartz, and specks of ilmenite altering into "leucoxene," are the 
only remaining minerals. 


The Archean rocks just described are all without question older than 
any of the Paleozoic series, which rest unconformably upon them; but 
of the relative age of these different components of the ancient crystal- 
line series it is in the nature of things difficult to form any very decided 
judgment. Even had time permitted a careful and detailed study of any 
of the remarkable exposures in the great glacial amphitheaters which have 
been carved out of them, it is doubtful whether their original relations 


could have been clearly made out, since they have been subjected not only 
to the dynamic movements which brought about the present elevation of 
the range, but, no doubt, to many previous movements of which no record 
now remains. As a consequence they are found to be contorted, fissured 
and reconsolidated, and fissured again, and this action seems to have been 
more intense the further one goes from the original surface, or rather from 
that which was the surface at the commencement of Paleozoic deposition. 
In general, it may be said that the pegmatites are the latest formations in 
the Archean proper, leaving out of consideration, of course, the later erup- 
tives (porphyries, porphyrites, and diorites) and that gneiss must certainly 
have formed part of the original undisturbed mass, while of the granites 
proper some were earlier and some later, but all previous to the pegmatite. 

On the accompanying plate (Plate IV) are reproduced a few hasty 
field-sketches of occurrences in which the different varieties of rock are 
found so intimately interlaced as to afford some idea of their relations and 
of the difficulty of tracing a sequence in their formation. 

In Fig. 1, it is seen (1) that across the original gneiss a small feldspar 
vein has been formed, probably the filling of a small fissure or crack result- 
ing from dynamic movement; (2) that the fine-grained and probably erup- 
tive granite has been intruded in tongue-like masses into the gneiss after 
the formation of this first vein; (3) that after consolidation the mass has 
again been shattered, a great fissure formed and filled by a coarser-grained 
granite, which surrounded fragments of gneiss a'nd earlier granite alike ; this 
fissuring was accompanied by a certain amount of faulting; (4) a second 
opening on the wall of this fissure has been made and filled with pegmatite. 

In Fig. 3, again, fragments of gneiss are found in a mass of fine-grained 
granite, in such position as to show that the latter must undoubtedly have 
been a more or less fluid mass, which traversed the gneiss and caught up 
included fragments of it in its passage. 

In Fig. 2, on the other hand, this granite is seen to have been sub- 
jected to at least two movements; as a result of the first, narrow feldspar 
veins have been formed across its mass, and again, by the second, these, to- 
gether with the inclosing granite, have been successively opened along the 
same fissure to admit the formation in fissures thus made of pegmatite 






Julius Bieii& Co. lith. 

f., Geologtat-in ' 



veins. The curving form of the smaller feldspar veins would also suggest 
an intermediate compression, during which the granite became sufficiently 
viscous to admit of some movement within its mass without producing 
fracture, for it is fair to assume that these veins are the filling of a crack 
along a fracture plane, and therefore originally more or less straight. 


The sedimentary deposits later than the Archean which are found in 
this region belong, with the exception of certain very recent beds, to the 
Paleozoic system. In the multitudinous sections afforded by the expos- 
ures along the cliffs of amphitheaters and the walls of canons remarkable 
uniformity in the physical characteristics of these beds is observed. Prac- 
tically the same bed, a fine-grained conglomerate, is, with a single excep- 
tion, found in contact with the underlying Archean wherever the contact is 
exposed and no non-conformity of stratification or other evidence of a phys- 
ical break exists. 

In determining the geological age of the different strata included in 
these series two difficulties are met at the outset : v first, the rarity of fossil 
remains in the beds, due probably to their relatively metamorphosed and 
altered condition; second, the absence of any systematic description of the 
Paleozoic horizons of the Rocky Mountain region, to be found in the pub- 
lished works of other geologists. The voluminous reports of the Hayden 
Survey contain, it is true, many local sections of sedimentary rocks and 
frequent surmises as to their age, but as yet, unfortunately, a systematic 
summary which shall correlate the material thus gathered by many differ- 
ent individuals into a harmonious whole, and sift out that which is to be 
considered fact from that which is only surmise, is wanting. 

It has long been the opinion of the writer, and one which is confirmed 
by later geological investigations, that it is impracticable to determine by 
similarity of molluscan fauna alone the correspondence of beds and forma- 
tions in regions so widely separated as are the Rocky Mountains, where as 
yet meager data have been gathered, and the Eastern States, where pale- 
ontological horizons are firmly established. In Paleozoic times these regions 
were practically two distinct continents, arid the conditions of life must have 
varied considerably. Until, therefore, the sequence of development and of 


extinction of molluscan life in the former region shall have been thoroughly 
investigated by detailed paleontological determinations, founded upon accu- 
rate and systematic stratigraphical studies, the assignment of geological hori- 
zons must be somewhat provisory and considerable importance must be given 
to the conditions of deposition which prevailed during the Paleozoic era. 

Geologists have observed, both in the East and in the Rocky Mountain 
region, a certain general sequence in the character of the sediments deposited 
in the oceans of former geological periods. This sequence has received 
from Dr. J. S. Newberry the name of "circles of deposition," and in a mem- 
oir on this subject he has endeavored to prove that in the Appalachian 
system each great geological period consisted of two extremes, during which 
the oceanic conditions were such that calcareous sediments were deposited, 
separated by an intermediate period, during which silicious sediment pre- 
vailed. The former, in a general way, are supposed to have occurred in deep 
seas and under conditions of comparative quiet, while coarser silicious sed- 
iments were formed either in shallow waters or during periods when this 
coarse material would be carried further out towards the middle of the 

As regards the assumption that limestone may be considered an evidence 
of deep-sea deposition, it seems that this evidence can be considered only as 
relative. The limestone depositions in the region under consideration, for 
instance, were formed in an inclosed arm of the sea, not more than 40 miles 
in width, and which can therefore have had no very great depth. Mr. John 
Murray, geologist of the Challenger expedition, informed the writer that 
the result of their investigations had been to prove that no limestone could 
be formed in the greatest depths of the ocean, and that the area of sedi- 
mentation is confined to a comparatively shallow and limited belt along the 
shores of the present continents. While it is probable, therefore, that none 
-of the deposits of the Rocky Mountain region were formed in seas at all 
comparable in depth to what are classed as deep seas by ocean explorers, 
the alternations of prevailing silicious and calcareous material in the sedi- 
ments doubtless represent significant changes in the oceanic or climatic con- 
ditions which prevailed to a greater or less extent over the whole region. 
It is, therefore, instructive to observe the parallelism of these conditions in 


the Paleozoic section of the Wasatch Range, as determined by the geolo- 
gists of the Fortieth Parallel and which was considered by them as the key- 
section of the Rocky Mountain region, and that of the Mosquito Range. 

In the former the Paleozoic series has a thickness of about thirty thou- 
sand feet and is characterized by two great silicious series, the Cambrian 
at its base and the Weber Quartzites in the middle of the Carboniferous. 
The former had a thickness of about twelve thousand feet and was followed 
by 1,000 feet of Silurian limestone, which was again succeeded by quartz- 
ites and sandstones of equal thickness ; this was followed by a great lime- 
stone formation of a maximum thickness of 7,000 feet, in the lower portion 
of which were found Devonian and Waverly forms, the main body of the 
limestone being, however, characterized by fossils of Carboniferous age. 
The coarse sandstones of the Weber series, which were deposited over this 
limestone, had a thickness in the Wasatch of about six thousand feet, and 
were succeeded at the close of the Carboniferous by alternating silicious, 
calcareous, and argillaceous beds. Followed eastward along the forty-first 
parallel, the whole Paleozoic series thins out rapidly, and in the Laramie 
hills, on the meridian of the Colorado or Front Range, seems to be rep- 
resented by a thickness of only 1,500 feet of rocks, though the exposures 
are not sufficiently good to render it certain that the entire series is here 

In the Mosquito Range the Paleozoic series has a maximum thickness 
of less than five thousand feet. The Cambrian is represented by quartzites, 
passing gradually upwards into calcareous shales, with limestones of prob- 
able Silurian age above, the aggregate thickness of the two being about 
four hundred feet. Above these limestones, and separated from them by a 
thin bed of quartzite, is the Blue, or ore-bearing, limestone, about two hun- 
dred feet in thickness, in which only Carboniferous forms have yet been 
found. This is succeeded by a relatively large development of silicious 
material, consisting mainly of coarse sandstones and conglomerates, corre- 
sponding lithologically to the Weber series, which passes upward into beds 
containing a greater or less development of limestone, with sandstones and 
shales, and which has been provisorily designated the Upper Coal Measures. 


Of the existence of the Devonian, which is recognized in the Wasatch 
section, and which was also found by Mr. Walcott in the Kanab, in the Colo- 
rado Plateau country, no direct evidence was found in the Mosquito region. 
On the one hand there is a gap of two hundred feet or more of beds from 
which no fossils were obtained, between the horizons in which Carboniferous 
and Silurian forms, respectively, were recognized. On the other hand, at 
one point evidence of non- conformity by erosion was observed between the 
Blue Limestone or base of the Carboniferous and the Parting Quartzite or 
top of the Silurian. Had this evidence of erosion been generally observed 
throughout the region, it would have afforded sufficiently conclusive proof 
that, owing to a perhaps local elevation, no sediments had been deposited 
here during the Devonian period. As it is, the question must remain for the 
present undecided, though the probabilities are in favor of the latter so- 

As to the existence or non-existence of the Devonian on the eastern 
slopes of the Rocky Mountains in general, the evidence is equally unsatis- 
factory. Waverly forms, which are associated with it in the Wasatch, 
have been found in the limestones of Lake Valley, in New Mexico. It is 
indicated on the Hayden maps as occurring on the south slopes of the San 
Juan Mountains, and Dr. Endlich's description of the formations in the 
neighborhood of the Anirnas River would seem to indicate the existence of 
a considerable thickness of beds below the Carboniferous which are not 
like the Silurian or Cambrian formations of Colorado in general. Unfor- 
tunately the fossil (Rhynconella Endlichi *) upon which he mainly founded 
his determination of the existence of Devonian beds in the region, has, upon 
recent, more careful study by Prof. R. P. Whitfield, been decided to be a 
Carboniferous and not a Devonian type. 

1 Geological and Geographical Survey of the Territories, 1874, p. 213. 



In the following tables are given the average Paleozoic section in the 
Mosquito Range, in the Kanab from Mr. C. D. Walcott, 1 and in the 
Wasatch from the Fortieth Parallel Reports: 

Mosquito section; 4,600 feet; possible unconformity by erosion. 

3,700 feet 

4,200 feet. 

Upper Coal Measures. 

Weber Grits. 

Weber Shales 

Bine Limestone 

f| Parting Quartzito 

Silurian J ! White Limestone 

200 feet. I j 
Cambrian Lower Quartzite 

200 feet. 







Blue and drab limestones and dolomites, with 
red sandstones and shales. Mud shales at 

Coarse white sandstones, passing into conglomer- 
ates, and silicionsaud highly micaceous shales, 
witb occasional beds of bliicli argilltto and blue 
dolomitic limestone. 

Calcareous and carbonaceous shales, with quartz- 

Compact, heavy-bedded, dark-blue dolomitic lime- 
stone. Silicious concretions at top, in form of 
black chert. 

White quartzite. 

Light-gray silicions dolomitic limestone, with 
white chert concretions. 

White qnaitzite, passing into calcareous and ar- 
gillaceous shales above. 

Kanab (Colorado River) section; 5,000 feet; unconformities by erosion. 



with impure sluily limestone at base. 

855 feet. 




1 455 

ceous bed, passing down into calcif PIOUS sand- 

Carboniferous ..- 
3,260 feet. 

Red Wall Limestone.. 


more massive and compact sandstone below. A 
few fillets of impure limestone Intercalated. 
Arenaceous and cherty limestone 2.15 feet, with 
massive limestone beneath. Chi-rty layers co- 
incident with bedding near base. 


100 feet. 


Massive mottled limestone, with 50 feH sandstone 
at base. 

785 feet. 


Green arenaceous and micaceous shales 100 feet 
at thu base. 

. Planes of unconformity by erosion denoted by double dividing lines. 
1 American Journal of Science, September, 1680, p. 222. 



W Match section ; 30,000 feet; conformable. 


Permian . ... 


Clays, iiiiifN, and limestones, shallow. 

650 feet. 

Upper Coal Measure 


Blue ami drab limestones, passing into sand- 



Weber Quartzite 


Compact sandstone and quartzite, often reddish; 

Carboniferous . . 

intercalations of limestone, argillites, nud con- 

14,350 feet. 


Wasatch Limestone 


Heavy -bedded blue and gray limestone, with sili 

emus .f'ini \i n: r I-IMTUIU near the top 

"Waverly. - - 

Devonian C 

2,000 feet. * 

Ogden Quartzite 


Puie Quart zile, willi cMiulumemte. 


Uto Limestone 


Compact or slialv silirinus limestone. 

1.000 feet. 




Silicious Hcbi>t* ami quartzite. 

12,000 feet. 


Lower Quartzite. The beds assigned provisorily to this horizon, which are 
indicated on the map in a dark-purple color (6), are prevailingly of quartz- 
ite. To them, therefore, the local name of Lower Quartzite has been given. 
Their average thickness is about one hundred and fifty feet to two hundred 
feet, of which the lower one hundred feet are composed of finely and rather 
thinly bedded white saccharoidal quartzites, while the upper fifty feet are 
shaly in character and more or less argillaceous and calcareous, passing by 
almost imperceptible transition into the silicious limestone of the Silurian 
formation above. 

At the very base of the series, at the contact with the underlying Archean, 
wherever this could be observed, is found a persistent bed of fine-grained 
conglomerate, from a few inches to a foot in thickness, made up of rounded 
and finely polished grains of bluish translucent quartz, generally not larger 
than a pea in size. Above this is a white quartzite of remarkably uniform 
and persistent character, always very readily distinguishable as a white 
band in the numerous sections offered by the canon walls of the range. Its 
thickness, when measured on the west side of the range, or near the Sawatch 
island, is, as mentioned above, 100 feet of purely silicious beds. On the 
east side of the range the thickness seems somewhat to diminish, and in 
places was found to be only 40 feet. 

In Buckskin canon a thin bed of silicious limestone was found included 
in the quartzite. The rock of this bed is remarkable as containing rela- 


lively a smaller proportion of carbonate of magnesia than any other lime- 
stone of the range, the specimen analyzed having 

Carbonate of lime 25.43 

Carbonate of magnesia .... 4.03 

The whole series may often be observed to be divided into two equal 
parts, the lower half consisting of very pure white quartzite, while the 
upper half weathers brown and is more or less stained by iron oxide and 
other impurities. 

While the lower series is very persistent in its character, the upper 
portion or transition series, which has a maximum thickness of 100 feet, is 
extremely variable, and, though readily recognized in all cliff sections, often 
seems to be wanting in those afforded by the numerous drill-holes in the 
neighborhood of Leadville. 

Owing to their similar lithological character and to the general absence 
of fossil evidence, it is difficult to establish a hard and fast line between this 
and the succeeding formation above. In practice the line has been drawn 
at the top of the shaly beds and the commencement of the beds of more 
massive limestone. The transition beds consist essentially of alternating 
bands of calcareous quartzite and shales. The name Sandy Limestones is 
often applied to them for the reason that on weathered surfaces of the cliff 
faces they appear like sandstones, the carbonate of lime having been entirely 
washed out and only the fine quartz grains left on the thin surface crust. 

One especially persistent bed of sandy limestone, generally about a 
foot in thickness, is often very useful in determining the horizon, on account 
of the striking appearance of its weathered surface. It is a silicious dolo- 
mite, generally of whitish color on fresh fracture, containing spots of dark 
brick-red resembling casts of fossils; for which reason the name Red-cast 
beds has been given to it. Fig. 1, Plate V, the reproduction of a photo- 
graph of a weathered specimen, shows its characteristic appearance. 

Certain of the shaly beds are found to contain a considerable develop- 
ment of pyroxene and amphibole, which often give a decided green color 
to the rock. The microscope shows besides an admixture of fine ore parti- 
cles, and in some cases there is so large a concentration of pyrites as to 
constitute veritable ore bodies. 


One of the most interesting features of this series is the local develop- 
ment of serpentine, resulting evidently from the metamorphism of pyroxene 
and amphibole. It has been found in small quantities at various points, but 
is developed on a very considerable scale in the Red Amphitheater in Buck- 
skin gulch, where it forms a remarkably beautiful verd-antique and a pecul- 
iar massive yellow rock, resembling bees-wax not only in color but also in 

Fossils. The only fossil remains found in this series occur in a bed of 
greenish chloride shales on the east flank of Quandary Peak, about a mile 
above the Monte Cristo mine. They belong to the genus Dicellocephalus, 
and resemble closely Dicellocephalus Minnesotensis of the Potsdam formation. 

Owing to the thick covering of forest immediately east of the point 
where these fossils were found, it was impossible to fix with absolute cer- 
tainty the exact horizon of the bed in which they occur. They are imme- 
diately above a heavy white quartzite, and beneath a bed of white marbleized 
limestone, which is in turn overlaid by the quartzite which carries the Monte 
Cristo ore deposit. From analogy with other sections, however, it seems 
safe to assume that it occurs above the main body of quartzite and near the 
base of the transition series. 


The beds assigned to this horizon consist of light-colored, more or less 
silicious, dolomitic limestone, capped by beds of quartzite of varying thick- 
ness which mark the dividing line between it and the overlying formation. 
On the general map of the Mosquito Range the entire series is included in 
one color-block (c). On the more detailed maps two divisions are made, 
to which the local terms White Limestone (c) and Parting Quartzite (d) 
have been given. 

white Limestone. The beds to which this local name has been given, 
from their prevailing light color as distinguished from the dark blue-gray 
or even black limestone above, consist in the main of light drab dolomites, 
and contain, besides the normal proportions of carbonates of lime and 
magnesia, from 10 per cent, upwards of silica. They are generally rather 
thinly bedded, of compact rather than crystalline structure, and frequently 
have a conchoidal fracture, approaching a lithographic stone in texture. 



Red Cast Beds .(Cambrian) 

Contorted Lime stone, (Upper Coal Measure.) 


But rarely do the beds have the whiteness of marble, and in such cases it 
is evidently due to local inetamorpliism. 

The characteristic feature of this limestone is the occurrence at certain 
horizons of concretions of white, semi-transparent chalcedony or chert. This 
occurrence is often useful in the mines of Leadville for distinguishing beds 
of this horizon from locally bleached limestones of the Carboniferous. Chert 
also occurs in the latter beds, but is always of dark, nearly black color, and 
the microscope shows in them a very finely granular structure, while those 
of the Silurian have frequently a radiate structure in the nature of spheru- 
lites. In neither was it possible to detect any trace of the minute organisms 
found in similar concretions in many other limestones. 

The average thickness of the White Limestone is from 120 to 160 feet. 
A small percentage of chlorine can be detected in these, as in all the other 
limestones from this region which were chemically examined. 

Parting Quartzite. Above the White Limestone occurs a bed of remark- 
able persistence, but of rather variable thickness, to which the above local 
name has been given, and which, on somewhat negative evidence, is regarded 
as constituting the upper limit of the Silurian formation in this region. In 
the cliff sections it has an average thickness of 40 feet, in one case attaining 
a maximum of 70 feet. It does not differ lithologically from the numerous 
white quartzites found at other horizons, but it is of geological importance 
as determining the dividing line between the Silurian and Carboniferous 
groups. In the cliff sections a brecciated structure is often observed in the 
limestone immediately overlying it, and in one case, on the east fork of the 
Arkansas, evidence of non-conformity by erosion was observed, which ren- 
ders it possible that the Upper Silurian and Devonian formations may be 
entirely wanting in this region. 

Fossils. Paleontological evidence as to the age of the above formation 
is extremely meager. No form was actually found in place. Casts of a 
Rhynconella, between R. neglecta and R. Indianensis of the Niagara epoch, 
were found in a prospect shaft in California gulch, not far from the White 
Limestone quarry, in such a position that they must have been derived from 
the beds of this horizon at least fifty feet above the base of the formation. 
Besides this, other specimens were brought in, obtained from talus slopes at 


the foot of the cliffs in Dyer Amphitheater and on West Sheridan, whose 
matrix of light drab-colored limestone renders it reasonably certain that 
they were derived from some of the beds of this horizon. The following 
forms are recognized : Leptena melita and an Orthisina like 0. Pepinensis, 
which correspond to forms found in the Calciferous ; and the syphon of an 
Endoceras, which belongs to the Trenton epoch. 

Corresponding beds in Colorado Range. In Order to obtain, for purposes of 

comparison, a section of the Paleozoic beds lying directly on the Archean 
along the Colorado Range uplift, a visit was made by Mr. Whitman Cross 
to the exposures in Williams cafion, near Manitou, and in Manitou Park. 
Although only fifty to seventy-five miles distant from the Mosquito Range 
exposures, the beds were found to vary so much in lithological composition 
that it was impossible to obtain an exact correspondence of horizons. The 
purely silicious beds at the base' are much thinner than in the Mosquito 
Range, the greatest thickness found being 50 feet. They are succeeded by 
calcareous sandstones and shales of variegated colors, red prevailing, which 
pass up into white or drab limestones, sometimes containing chert secretions 
and alternating with shaly beds, with an aggregate thickness of about two 
hundred feet. These beds may be considered as the equivalents of the 
Lower Quartzite and White Limestone of the Mosquito Range. Owing to 
extensive denudation it was impossible in the time allotted to trace a con- 
tinuous series into well-defined Carboniferous horizons. 

From the east bank of Trout Creek (Bergens Creek on the Hay den 
map), in Manitou Park, two miles below the hotel, Mr. Cross obtained fossils 
which have been identified by Mr. C. D. Walcott as follows : 

From reddish-brown sandstone 45 feet above tbe Arcbean. 

Lingulepix, sp. ? Au elongate form allied to L. pinnmformis of tbe Potsdam sand- 
stone of Wisconsin. 

From red calcareous sandstones, alternating with white limestone, one hundred 
and five to one hundred and twenty-two feet above the Archeau. 
Glytocistites (?). Single plates. | CyrtolHes. 

Lingula, sp. undet. ; probably new. 
Orthis desmopleura, Meek. 

Ortlwceras, sp. undet.; probably new. 
Bathyurus simillimus, Walcott (f ). 

Metoptoma, new sp. 

This fauna is essentially the same as that of the upper third of the Pogonip Lime 
stone of Nevada. 


The paleontological information, therefore, is so far a confirmation of 
the suggestion offered above from lithological composition, viz, that the 
Cambrian beds are here not more than fifty to a hundred feet thick (a 
notable decrease from the estimated 12,000 feet in the Wasatch, or from the 
more definitely-determined thickness given by Mr. A. Hague for Eureka, 
Nevada, of 7,700 feet), and that the limestone beds above are Silurian. 


The beds of this period are, as in other parts of the Rocky Mountain 
region, more fully developed and more abundant in fossil remains than 
those of the other Paleozoic horizons. The Carboniferous period here, as 
in the Wasatch, consisted of two limestone-making epochs, separated by a 
long period of silicious deposits, with the difference that in the shallow seas, 
in which the Carboniferous of the Mosquito Range was formed, detrital and 
silicious deposits predominated over calcareous deposits. The series, there- 
fore, lends itself to a triple subdivision into lower, middle, and upper Car- 
boniferous, which are here assigned to it mainly on lithological grounds, since 
our knowledge of the Carboniferous fauna of the Rocky Mountain region 
is not yet sufficiently complete to enable us to establish satisfactory paleon- 
tological subdivisions, and many forms considered characteristic of the Coal 
Measures of the East range from the bottom to the very top of the series. 

Blue or ore-bearing Limestone. The beds included under this local name, 
which are designated on the map by a deep-blue color (e), and which, from 
the fact that they form the ore bearing rocks par excellence of the region, it 
is most important to be able to trace accurately, are fortunately marked by 
persistent and characteristic features. They have an average thickness of 
about 200 feet, In color they are of a deep grayish-blue, often nearly 
black in the upper portion of the series, while some of the lower beds are 
lighter in color, approaching a drab, and, where locally bleached, difficult 
to distinguish lithologically from the underlying White Limestone. The 
upper bed is well marked by characteristic concretions of black chert, fre- 
quently hollow in the center and often containing within their mass dis- 
tinct casts of fossils. Owing to their superior resistance to atmospheric 
agencies, they are often weathered out and left in nodular masses of irreg- 


ular shape upon the surface. The forms which they assume are sometimes 
so fantastic as to suggest to the untechnical that they are the fossil remains 
of some gigantic animal. Their forms, however, are always rounded, and 
are more commonly that of a sphere or some solid of revolution. In many 
cases, that they are the filling in of a pre-existing cavity in the limestone 
is evident from the fact that they are hollow in the center and contain 
crystals of pyrite or other minerals lining the cavity. 

The series is generally heavily bedded, and the rock is almost always 
granular, and in the upper part often coarsely crystalline. A character- 
istic feature, especially of the upper portion of the formation, is a ribbed 
structure produced by irregular lines and spots of white crystalline mate- 
rial. In some cases the ribbing is so fine and regular as to produce an ap- 
pearance resembling that of the Eozoon. 

This typical appearance of the rock is shown in Plate VI, on which 
are represented two specimens from the Blue Limestone of Iron hill, taken 
a short distance below the ore body on the Silver Wave claim, which were 
also subjected to microscopical examination. The upper figure in the plate 
is a photograph of a specimen polished on one side to show the fine ribbing 
which is peculiar to this limestone. The lower figure shows a specimen 
roughly shaped by the hammer, in which tire ribbings or veins of white 
crystalline spar are coarser and more irregular. These white crystalline 
veins may be supposed to be produced by the dissolving out of a portion of 
the limestone and its redeposition in a crystallized form. As bearing on 
the question of the relative solubility in natural waters of carbonates of 
lime and magnesia, a partial analysis of the white spar was made, and it 
was found to have the same proportions of the two salts as the dark granular 

Composition. The composition of the rock, which is remarkably uniform, 
is that of a normal dolomite, the average of six lime and magnesia deter- 
minations from different localities giving 

Carbonate of lime 54.695 

Carbonate of magnesia 43.197 

the proportion in normal dolomite being 

Carbonate of lime 54.30 

Carbonate of magnesia 45.70 



Blue Limes-tomes 

' 211 I'K'ILCIIT.f 



The following complete analyses of typical specimens, taken from local- 
ities at considerable distances from each other in the vicinity of Leadville, 
are further proofs of the uniformity of composition. I, II, and III are 
from the upper of the Blue Limestone, IV from near its base, and V from 
the upper part of the White Limestone. 






Silver Wave 



Carbonate hill 








(Guyard ) 

(Guyard ) 

(Hillebrand ) 

30 79 

30 43 

29 97 

27 26 

26 60 

21 14 

20 78 

21 52 

20 05 

17 41 

46 84 

46 93 

47 39 

43 79 

40 01 










1 51 









] 66 






11 84 






I'ntitsli , .... ,.. 







0. 062 



























100 018 

99 925 

100 006 

100 436 

The coloring matter is in part evidently organic, but iu part, as sug- 
gested by Mr. Guyard, may be due to the presence of salts of iron. He 
says that he finds an appreciable amount of sulphide of this metal which will 
produce a black color. A remarkable feature in this analysis, as well as 
in that of the White Limestone, is the presence of appreciable quantities 
of alkaline chlorides. Microscopical examination under very high power 
(1,136 diameters) shows that the dusty appearance is due to minute specks 
in the grains composing the rock, which are fluid inclusions, in some of which 
the rapid movement of a bubble is visible. As will be shown later, it seems 
fair to assume that the included liquid consists of alkaline chloride. The 
microscope also shows that the rock is very finely granular, the size of 
the grains varying from .05 to .10 of a millimeter in diameter. No twin 
crystals of calcite are observed, and very little quartz or ore particles could 
be detected. 



The characteristics which may serve in the field to distinguish the rock 
of the Blue from that of the White Limestone are as follows : 

1 . Color, which is darker. 

2. Composition, the former being almost free from silica, the latter con- 
taining 10 per cent, and upwards. 

3. Texture, the former being generally crystalline, while the latter is 
more compact. 

4. Chert secretions, which in the former are always black and in the 
latter nearly white. 

5. Structure, the Blue Limestone being generally more heavily bedded 
than the White. 

Fossils. The only fossils obtained from this horizon were found in the 
extreme upper part of the formation, either in the limestone itself or in 
chert nodules, which are found scattered over its weathered surface. The 
following' forms were obtained from five different localities : 

Euomphaliis, closely resembling E. Spergenensis, Hall, from Warsaw limestones 
of Spergcii hill. 

Spiriferina, which is probably new, though somewhat resembling 8. Kentuckensis. 

Athyris siibtilita. 

Pleurophorux oblongus. 

Products coztatiis. 

Spirifera (Martinia) lincata. 

Spirifera Rockymontana. 

Streptorhynchm crassus (crenistria). 

Cyathopltylloid corals, resembling Zaphrentis. or Cyathaxonia cynodon. 

While most of these forms are common to the Coal Measures of the 
East, the first-mentioned is there found in the Lower Carboniferous. For 
this reason and because this form and the Spiriferina do not occur in any 
of the higher beds, it seems justifiable to assume that this horizon represents 
the Lower Carboniferous of this district. 

The upper limit of this formation has been fixed at the top of the 
massive Blue Limestone, which is generally marked by the frequency of 
chert concretions, and in the mining districts has been followed by prefer- 
ence by the ore-bearing solutions. Locally, however, limestone formation 
seems to have continued somewhat intermittingly for some distance above 
this horizon. 


Weber shales. On the general map of the Mosquito Range, owing to its 
small scale, it was considered advisable to make no subdivisions of the 
Weber Grits formation, and the whole is therefore included under one color 
(ff). On the more detailed maps, however, a subdivision of the Weber Grits, 
designated the Weber Shales, has been distinguished by a distinct color (/). 
The beds included under this name are extremely variable in lithological 
character and in thickness. They constitute a transition series between the 
massive limestones below and the characteristic coarse sandstones of the 
Weber Grits above. They consist of argillaceous and calcareous shales 
alternating with qnartzitic sandstones. The former are generally carbona- 
ceous, and in their extreme type pass into an impure anthracite. The cal- 
careous shales, on the other hand, are locally developed into a considerable 
thickness of impure limestone, which is very rich in fossil remains. Owing 
to its variable character and to the fact that the dividing plane between 
this and the preceding is frequently occupied by beds of porphyry, it is 
difficult to assign a definite thickness to the formation. It may, however, 
be assumed as varying from 150 to 300 feet. 

In Leadville itself a thin bed of quartzite is often found immediately 
above the Blue Limestone, and on Iron hill is a greenish argillaceous shale, 
called the Lingula shale, from the abundant casts of this fossil which it 
contains. The coal development attains a thickness in one case of seven 
feet, but is extremely impure and gives little promise of any economical 

Fossils. The most common form is Lingula mytiloides, Meek, which is 
supposed to correspond to L. ovalis, Sowerby. Besides these were obtained 
from several different localities the following : 
PMllipsia, sp. ? (P. major?) 

Productus cora. 
Productus semireticu latus. 
Productus pertenuis. 
Productus muricatus. 
Productus Nebrascensis. 
Spirifera cameratus. 
Aviculopecten rectilaterarius. 
Orthis carbonarius. 
Ktreptorhynchus crassus (crenistria). 
Chonetes granulifera. 

Discina nitida. 
Macrocheilus ventricosHS. 
Eoccldaris Halliana. 
Fenestella perelegans. 
Rhombopora lepidodendroides. 
Myallna pcrattenuata. 
Polyphemopsia, (like P. chrysalis). 
Pinna , sp. .' 
Polypora, sp. mulct. 
Palceschara, sp. uudet. 


Weber Grits. This formation, which, as its name implies, consists mainly 
of coarse sandstones passing into conglomerates, has an estimated aggregate 
thickness of 2,500 feet, although neither its upper nor its lower limits can 
in the nature of things be very sharply defined. 

The typical rock, which often forms massive beds of considerable thick- 
ness and constitutes a prominent feature in the sections afforded by canons, 
is a coarse white sandstone passing into a conglomerate, made up of well- 
rounded grains and pebbles, mainly of white and sometimes of pinkish 
quartz. In the coarser conglomerates feldspar can often be distinguished in 
fragments, and this mineral is often disseminated in fine grains throughout 
the sandstone, but fragments of recognizable Archean schists are not often 
seen. It would seem, therefore, that these beds are mainly formed by the 
abrasion of the coarser granites of the Archean. The sandstones often contain 
a considerable admixture of brilliant white mica, and in some cases, besides 
the mica, so large a quantity of carbonaceous material as to become quite 
black. This carbonaceous material, which is insoluble in ether, alcohol, or 
sulphide of carbon, is probably either graphite or anthracite. 

Next to the sandstones and conglomerates, the most important constitu- 
ents of the formation are quartzose shales and mica schists, generally coarse- 
grained and of a greenish hue. Their lamination is very regular and often 
parallel to the bedding-planes, so that they often weather out in slabs or 
flags of considerable size. The mica, which, as in the sandstones, is mostly 
potash mica or muscovite, seems to form but a subordinate part of the rock 
mass, but is generally very prominent in large brilliant flakes on the surfaces 
of the laminae. Microscopical examination shows that in the sandstones 
and schists feldspar is always present with the quartz, and in some cases the 
three varieties, orthoclase, plagioclase, and microcline, can be distinguished. 
It also shows that the muscovite is, in part at least, derived from the decom- 
position of the feldspars; at the same time the uniform occurrence of large 
brilliant flakes along the bedding-planes of the shaly material suggests the 
possibility that these may have been directly derived from debris of the 
Archean and have been deposited in this position by the action of water. 

At irregular intervals throughout the formation are found beds of fine 


black mud-shales or carbonaceous argillites, generally very thin and some- 
times calcareous, passing into impure limestones. 

About the middle of the formation is a tolerably persistent develop- 
ment of limestone of the usual blue-gray color and dolomitic in composi- 
tion. Its thickness, however, varies very much according to locality. It 
was best observed in Big Sacramento gulch, a short distance above the Lon- 
don fault, where are two beds of limestone with associated shales, about 
fifty feet apart and each about ten feet in thickness. 

Fossils. From the limestones in Big Sacramento gulch were obtained 
the following forms: 


Spiriferina Eentuckensis. 
Athyris subtilita. 

Productus costatus. 

Productus muricatus. 
Aviculopecten interlineatus. 
MeeJcella stricecostata. 

From micaceous schists in the upper part of the formation between 
Lamb and Sheep Mountains were obtained abundant casts of Equisetacece. 

upper coal Measures (h). Less favorable opportunities were offered for 
studying this group than for either of the preceding, since its beds were 
found only at the extreme limits of the map and in regions where continu- 
ous outcrops are rare. It consists of alternating calcareous and silicious 
beds, the latter not being distinguishable from those of the Weber Grits 
at the base, but passing upward into reddish sandstones, which in their turn 
are sometimes difficult to distinguish from the overlying red sandstones of 
the Trias. Its lower limit is drawn at the base of the first important lime- 
stone bed above the Weber Grits. This limestone, locally called the Robin- 
son Limestone from the fact that it forms the ore-bearing horizon of an im- 
portant mine of that name in the Ten-Mile district, is remarkable for being 
the first true limestone observed among the calcareous beds of the region. 
All below this horizon are practically dolomites of varying purity. As 
developed in this mine, it is of drab color, conchoidal fracture, and of pecul- 
iarly compact texture, resembling a lithographic stone. Its purity and 
textural characteristics are apparently not persistent outside of the Ten-Mile 
district. In the upper horizons of this district are found mud-shales, resem- 
bling in lithological character the Permo-Carboniferous of the Wasatch. 
Their fossil remains are found, however, to be distinctly Coal Measure forms. 



The upper sandstones of this group are distinguished from the overlying 
Triassic rocks by a deeper color, approaching a Venetian red, whereas in 
the latter the color is rather of a light brick red. 

Plate V (p. 60) shows a remarkably contorted specimen of impure 
limestone of this horizon from the outcrops on Empire hill, where abun- 
dant fossils were found. 

Fossils. Fossil remains were found in various beds of this formation 
in the Ten-Mile district; in a peculiar black limestone of the Hoosier ridge, 
to the northeast of Mount Silverheels ; and on Empire hill, on the west 
side of the range, adjoining Weston fault. 

From ten different localities in these regions the following forms were 
obtained : 

Productus costatus. 
Producing Nebrascensis? 
Productus Prattenana. 
Productus cora. 
Spirifera Bockymontana. 
Spirifera (Martinia) lineata. 
Spirifera camerata. 
Athyris subtilita. 
Streptorhynchus crassm. 
Chonetes Glabra. 
Bellerophon crassus. 
Bellerophon percarinatus. 
Bellerophon (sp. ?). 
Microdon tenui-striatum (very small). 
Microdon obsoletum. 
Pleurophorus occidental IK. 

Pleurotomaria (like P. Qreyvillewis). 
Xaticopxiii (like y. Altonensis). 
Macrochcilw (primigenius ?). 
Nucula (tentricosa t). 
Nucula (like N. Beyriche). 
Microdoma (nearly M. conica). 
Euomphalus (sp. !). 
Archfeoccidaris (sp. t). 
Astartella (8p. ?). 
Loxomena (sp. t). 
Fenestella (sp. t). 
Murchinonia (sp. f). 
Synocladia (sp. t). 
Nautilus (sp. ?). 
Entolium (sp. t). 
Amplexm (sp. !) 


As Mesozoic beds do not occur within the area of the map, no attempt 
was made to study them systematically or to obtain a measurement of 
their thickness, which would have taken a great deal of time and probably 
been impracticable without a more detailed map than could be had, Their 
aggregate thickness has therefore been assumed to be not less than GjOOO 
feet, a safe estimate judging from the thicknesses given by the geologists 
of the Hayden Survey for various parts of Colorado. 


The red sandstones of Mount Silverheels, above the beds assumed to 
be Upper Coal Measures in this report, are noticeable for their coarse grain 
and for the abundant pebbles of Archean rocks which they contain. In 
some intercalated shaly beds just east of Fairplay, Professor Lakes found 
plant remains and fossil insects. The former were determined by Professor 
Lesquereux to be undoubtedly Permian and the latter by Mr. A. Hyatt to 
be as certainly of Triassic age. In such conflict of evidence it seems safer 
to trust to that of animal life, since it is already well established that in 
America plants came into existence in Cretaceous time which in Europe 
have always been considered to have made their first appearance during 
the Tertiary. 


The Quaternary formations which have been designated by special 
colors on the maps and sections are the Glacial or Lake beds, and the 
Post-Glacial or recent detrital formations. As already shown, there is 
evidence of the existence, during the intermediate flood period of the 
Glacial epoch, of a large fresh-water lake at the head of the Arkansas 
Valley, in whose bed was deposited a considerable thickness of coarse and 
rudely-stratified beds of detrital material from the adjoining mountains. 

Glacial or Lake beds (q). Owing to the limited opportunities afforded for 
obsei'ving these beds in place, it was impossible to obtain a complete sec- 
tion of them or an accurate estimate of their aggregate thickness. The 
maximum thickness observed is about 300 feet; their material is generally 
coarse, and, as might be expected, very much coarser along what is known 
to have been the shore line of the lake. The finest of the beds consist of a 
calcareous marl, whose development seems to have been extremely local. 
The prevailing beds are a loose friable sandstone, resembling granite decom- 
posed in place, consisting largely of grains of quartz and feldspar, and 
often somewhat iron-stained. These beds frequently alternate with those 
of coarser material, which form a rude conglomerate. The coarser beds 
contain both angular fragments and bowlders of the rocks which make up 
the range, and lithologically can hardly be distinguished from the Wash of 
the succeeding formation; but, where any considerable thickness of the 


beds is cut through, the stratification lines are easily recognizable and serve 
to distinguish this formation from the latter. 

Along the immediate shore-line as, for instance, under the Wash of 
Fryer and Carbonate hills the upper portion of the Lake beds consists fre- 
quently of large angular fragments, a number of which are derived from 
the actual outcrops of ore bodies. 

Recent or Post-Giaciai (r). Theoretically this rubric includes all the beds 
of the Post-Glacial Quaternary formations, of which there have been recog- 
nized 5 a the region under survey several subdivisions, namely: the glacial 
moraines, a sort of bowlder clay or rearranged moraine material which is 
prevalent in the immediate vicinity of Leadville, where it received the local 
name of " Wash ; " a sort of terrace formation found in the larger valleys; and 
the actual alluvial stream bottoms. 

The time allotted to the work did not admit of a sufficiently complete 
study of these different subdivisions to justify their distinction by separate 
colors on the map. In practice, therefore, on the surface maps only the 
alluvial bottoms and the broader accumulations of the terrace gravel in the 
larger valleys and plains, which are sufficient to completely obscure the 
subjacent geology, have been indicated. In the cross-sections of the spe- 
cial map of Leadville, however, where the explorations of shafts have given 
unusually complete data, the Wash is also included under this rubric. On 
the surface maps of Leadville and of the various groups of mines both 
these formations have been left out, as they would have hidden an impor- 
tant part of the geological outlines of the actual rock surface; they have, 
however, been indicated to scale in the cross-sections. 


The superficial distribution of the various sedimentary formations, or 
the relative area covered by their outcrops, being a function of or depend- 
ent upon erosion, is intimately connected with the existing topographical 
structure of the region. Were erosion the only factor to be considered, the 
Archean rocks would be found exposed continuously on the west side of a 
line approximately representing the old shore-line and in the deeper drain- 
age valleys and anticlinal axes of the eastern side. The displacements of 


the numerous faults which run through the region have, however, consid- 
erably modified this normal distribution. In point of fact, the central por- 
tion in the latitude of Leadville is mainly covered by the outcrops of Pa- 
leozoic sedimentary beds and of intruded masses of porphyry, the Archean 
exposures being confined to deep glacial amphitheaters near the crest of 
the range, and to minor masses which represent the eroded crests of anti- 
clinal folds. 

In the northern portion of the area Archean rocks are exposed along 
the main crest of the range and in the deep canon valleys and glacial am- 
phitheaters of the streams which flow into the Platte, Paleozoic beds being 
found only on the eastward sloping flanks of the included spurs. On the 
western side of the range, owing to the displacement of the great Mosquito 
fault, the area adjoining the valley of the east fork of the Arkansas is cov- 
ered by beds of the Weber Grits formation, while a bordering fringe of 
outcrops of Lower Quartzite and White and Blue Limestone beds is found 
on the northern and eastern rim of Tennessee Park. 

In the southern half of the map the western limit of Paleozoic beds is a 
line running southeasterly from the forks of the Arkansas to the crest of the 
range at Weston's pass, and southward beyond the limits of the map along 
the crest, approximately in a north and south line. West of this line are 
found only the granites and schists of the Archean, and irregular dikes and 
intrusive masses of porphyry. In the area included between this line and 
the crest of the range are triangular zones of easterly dipping sedimentary 
beds, in some cases forming a continuous series from the Cambrian to the 
Upper Coal Measures, cut off abruptly by fault-lines and succeeded again 
on the east by Archean exposures. On the east of the crest the Paleozoic 
beds slope regularly back beneath the floor of the South Park, the Archean 
rocks being found only in the deeper hollows at the heads of the streams 
Beyond the limits of the map the outcrops of the more resisting beds of 
Mesozoic age form parallel ridges, running across South Park from north to 
south. The Quaternary Lake beds are found only along the lower ends of 
the spurs extending out into the Arkansas Valley from Leadville south to 
the limits of the map. 



The eruptive rocks of this region, besides the granites, which were 
erupted during Archean time, are of Mesozoic or Secondary and of Tertiary 
age. The most important of these, both in magnitude of development and 
in their relations to the ore deposits of the region, are the Secondary erup- 
tives ; the time of their eruption cannot, as explained in the preceding 
chapter, be exactly fixed, but was probably toward the close of the Mesozoic. 
The Tertiary eruptives, on the other hand, are of comparatively limited de- 
velopment and have had no appreciable influence on the deposition of ore ; 
their age is determined as such, not by any direct crossing of Tertiary beds, 
of which no instances were found in the region, but from their lithological 
character, their analogy to eruptive rocks of known Tertiary age outside 
of this area, and from the fact that they are later than the Secondary erup- 


The earlier eruptive rocks occur mainly in the form of intrusive sheets, 
often of great magnitude, which, having been forced up from below through 
some more or less vertical vent or channel, have spread themselves out be- 
tween the strata, generally following a definite horizon, but at times crossing 
the stratification. They also occur in the form of dikes, this form being 
most common in the underlying Archean rocks. There is no evidence that 
any of them were poured out upon the surface like the lavas of the present 
day, but they must have cooled and consolidated under a great weight of 
superincumbent strata, to which is doubtless in great measure due their 
unusually crystalline character. 

They are with unimportant exceptions porphy ritic in structure ; that is, 
they contain larger crystalline elements in a groundmass or matrix of finer 
grain, as distinguished on the one hand, from the granitic structure, in which 
all the elements are crystalline and of comparatively uniform size, and from 
Tertiary eruptives on the other, in which, while the structure may be por- 
phyritic, the larger crystals have a somewhat different development and 
the groundmass is made up in great part of non-crystalline material. 


These distinctions are those that were in force before the introduction 
of the use of the microscope in lithological study. The more intimate 
knowledge of rock structure obtained by the microscopical study of rocks 
has brought about many changes in preconceived ideas, which are increas- 
ing every year, so that it seems merely a question of time as to when a new 
system of classification may be required. Already the distinctions noted 
above are true only of the most typical varieties of each, while between 
these are transition members which often must be placed in the one cate- 
gory or the other by some other distinguishing characteristic, such as time 
of eruption, internal structure, etc. In the present work it has been judged 
best to preserve the prevailing usage of designating the Secondary porphy- 
ritic rocks in which the prevailing feldspar is orthoclastic as porphyries, and 
those in which plagioclastic feldspars decidedly predominate as porphyrites. 
When the porphyrite is entirely granitic or evenly granular it becomes a 

On the general map of the Mosquito Range only two colors are given 
to the porphyries, founded on two general divisions which have a geograph- 
ical as well as a structural value. In the first of these is included the White 
Porphyry and its closely allied form, the Mount Zion Porphyry, which are 
the older and more nearly granular rocks, and which occur, with unimpor- 
tant exceptions, only south of the north line of the Leadville map ; the sec- 
ond includes all other varieties of the Secondary porphyritic rocks of the 
region, which are generally younger and less uniformly crystalline, and 
which do not occur south of the south line of the Leadville map. 

On the detailed map of Leadville and vicinity the principal varieties 
of porphyry are each designated by a special color, the division " Other 
porphyries " including those which could not, with absolute accuracy, be 
brought into either of the other divisions. 

1 In the time that has elapsed since field work was completed and the maps colored, opportunity 
has been had for studying more comprehensively the various Secondary eruptives in the course of 
work carried on in neighboring districts, and it has been found that some of the varieties designated 
oil the following pages as porphyry, viz, the Sacramento, Silverlieels, and Green porphyries, should 
probably be classed as porphyrites. The reasons for this, as well as the detailed description of all the 
rocks from a microscopical point of view, deduced from their study under the microscope by Mr. Cross, 
will be found in Appendix A. 



This porphyry, when fresh and unaltered, is a gray rock resembling 
fine-grained granite, and is made up mainly of quartz, feldspar, and mica ; 
orthoclase being the predominant feldspar and biotite the original mica; 
plagioclase feldspar is decidedly subordinate, and biotite but sparingly 
developed. It is rarely found in an unaltered condition, however, and in 
the various stages of alteration it passes through a rock in which the partly 
decomposed biotite produces a slightly spotted appearance into a white rock 
glistening with fine lustrous particles of muscovite which can hardly be dis- 
tinguished from the White Porphyry. The muscovite results mainly from 
the decomposition of the feldspar and also from that of the biotite. Larger 
individuals of quartz and feldspar, as porphyritic ingredients, can frequently 
be distinguished by the naked eye. Beside the above minerals the micro- 
scope also detects zircon, magnetite, and apatite as accessory constituents 
of the rock ; it shows, too, that the texture of the rock is quite gramilnr 
throughout, with no amorphous material. 

Occurrence. This rock is of comparatively limited development, being 
found thus far only on Mount Zion and on Prospect Mountain. It is gen- 
erally in a less altered and therefore more typical condition on Mount Zion, 
for which reason it has received that name; but the most entirely unaltered 
specimens were obtained from some deep shafts on Prospect Mountain. On 
the south slopes of Prospect Mountain it is generally very much decom- 
posed and apparently grades off into White Porphyry, so that it is difficult 
to draw a sharp dividing line between the two rocks. No rock that could 
be definitely classed with this variety has been found south of Evans gulch, 
and the body in the bed of the gulch above the mouth of South Evans has 
been assigned to it somewhat doubtfully. 


The White or Leadville Porphyry is a generally white or granular, 
compact, homogeneous-looking rock, composed of quartz, feldspar, and 
muscovite. The quartz and feldspar are so intimately mixed together that 
they can only occasionally be distinguished by the naked eye, the former 
in small, double-pointed, hexagonal pyramids, the latter in small, white, rect- 


angular crystals. The muscovite as an original constituent occurs in spar- 
ingly distributed, dark, hexagonal plates, which were at first supposed to be 
biotite; their true character was learned only when a specimen was found 
containing enough of the crystals to be subjected to optical and chemical 
tests. (See Appendix B, Table I, Analysis II.) A characteristic appear- 
ance of the rock is the frequent occurrence of pearly-white leaflets of mus- 
covite, often in star-like aggregations, resulting from the decomposition of 
the feldspars. Orthoclase is the predominant feldspar. No biotite has ever 
been detected in the White Porphyry; but, as the rock is always in a more 
or less advanced stage of decomposition and as biotite occurs in the Mount 
Zion Porphyry, which seems to pass into it, it may have been an original 
constituent, though it is rather remarkable that no traces of it exist even 
in the small dikes where the rock still retains a distinct porphyritic struct- 
ure and has a fresh conchoidal fracture. By means of the microscope are 
found zircon as a common and magnetite and apatite as rarer constituents 
of this rock. No glassy matter is found, either in groundmass or in inclu- 
sions. Chemical analysis shows an appreciable amount of BaO and PbO, 
substances common in the ores, in its composition. 

Among the miners it is known also as "block porphyry," on account of 
its tendency to split up into angular blocks, which are often stained interi- 
orly in concentric rings by iron oxide; and also as "forest rock," from the 
frequent deposition of dendritic markings of oxide of manganese on the 
cleavage surfaces. 

Occurrence. The principal development of the White Porphyry is con- 
fined to a zone about the width of the Leadville map, and running from the 
western boundary of that map south of east, instead of due east as the map 
itself does. In other words, its lines have the prevailing northwest and 
southeast trend of other larger features of the region. Within this zone it 
is developed on an enormous scale, and occurs mainly as an intrusive sheet 
directly overlying the Blue Limestone and in contact with the principal ore 
deposits. It is not, however, entirely confined to this horizon, but is also 
found at both lower and higher horizons and can sometimes be observed 
crossing a stratum, generally at a low angle, from one horizon to another, 
thus splitting the sedimentary bed into two wedge-shaped portions. This 


occurrence is most noticeable in the area of the Leadville map along an 
imaginary northwest and southeast line, on one side of which it is found 
both above and below the Blue Limestone, while on the other it occurs only 
above it. 

The main sheet has an average thickness of several hundred feet and 
varies in its extreme dimensions from 20 feet along the northeast edge of 
the zone to 1,500 feet at White Ridge, on the east side of the range, the 
point of its maximum development and supposed to be the locality of its 
principal vent. 

Although all these masses must have been originally forced up from 
below through the Archean, it is remarkable that no section has yet been 
found which would show the actual passage from the Archean dike to the 
interbedded sheet. The nearest approach to this has been at the head of 
Iowa gulch, on Empire hill, and in a Lore-hole in South Evans gulch, 
where White Porphyry has been found in the Archean in probable dike 
form, and on White Ridge and Lamb Mountain, in Horse Shoe gulch, where 
it is seen cutting up nearly vertically across Carboniferous strata. 

South of the zone above mentioned, White Porphyry is found as a 
remarkably persistent sheet at the Blue Limestone horizon gradually thin- 
ning out and extending to the southward as far as Weston's pass. North of 
the zone it is found only in small sheets at Little Zion, Mosquito Peak, and 
London hill, and in several small dikes in the Mount Lincoln massive, its 
place being occupied by other varieties of porphyry. 


The other forms of porphyry found (and which on the Mosquito map 
have been designated by one general color), though presenting a number of 
varieties in the field, have essentially the same general composition, both 
mineralogiccil and chemical. They consist mainly of quartz, two feldspars, 
and biotite, hornblende occurring as an essential ingredient only in one 
variety. The crystalline ingredients are easily distinguishable by the eye, 
and there is therefore no danger of confounding them in the field with 
White Porphyry, except in the conditions of extreme decomposition in 
which they may be found near the ore bodies. This crystalline structure, 


on the other hand, is often so far developed that they are not readily dis- 
tinguished by the untechnical eye from granites ; as such, indeed, they are 
frequently classed by the miners. A careful examination, however, readily 
reveals their structural difference, which is that in them the larger crystals 
are inclosed in a finer-grained groundmass, whereas between the crystals of 
granite there is no such intervening and apparently structureless material. 

The principal subdivision of this group has been called Lincoln 
Porphyry, from the fact that it is typically developed in the mountain mass 
around Mount Lincoln. Its most striking characteristic is the frequent 
occurrence of large crystals of pinkish orthoclase, from one inch upwards in 
size, with a peculiar luster like that of sanidine. Plagioclase is generally 
in small, white, opaque crystals. Quartz occurs in double-pointed hexagonal 
pyramids, which have a rounded outline on fracture surfaces and often a 
slightly roseate tint. Mica is found in small hexagonal plates, generally 
decomposed and of greenish color. The microscope discloses, in addition 
to the above minerals, allanite, zircon, magnetite, titanite, and apatite. No 
microfelsitic or glassy matter is found in any rock of this type and no glass 
inclusions occur in the Mount Lincoln rock. Orthoclase feldspar predomi- 
nates in the groundmass and in the rock as a whole, while among the 
porphyritic crystals of rocks, in which the characteristic large orthoclase 
are wanting, plagioclase is in relatively larger proportion. Owing to the 
size of the crystals, large masses of the rock have at a little distance a 
decidedly granitic appearance. On weathered surfaces, especially in the 
dry region of the mountain peaks, it is of light-gray color, somewhat 
bleached, and often slightly stained by hydrous oxide of iron. In mine 
workings, on the other hand, when freshly broken it has a decidedly greenish 
tint, from the change of biotite into chlorite. 

occurrence. The main development of the typical Lincoln Porphyry is 
in the neighborhood of Mount Lincoln, where it occupies the same position 
with regard to the ore deposits of that region that the White Porphyry does 
about Leadville. It forms the immediate summit of Mount Lincoln, where 
it is apparently the remains of a laccolitic body or head of a channel of 
eruption. It occurs as an interbedded sheet in the Cambrian and forms 
several large bodies, apparently interbedded sheets, in the Weber Grits 


which form the wooded ridges on either side of the Platte Valley in that 
region. It also occurs in the form of narrow dikes, cutting through the 
Archean. On the west side of the range it forms many large bodies in the 
Weber Grits, the most important of which is the laccolite body of Buckeye 
Peak. These bodies in the northwestern part of the region pass into the 
closely allied variety called Eagle River Porphyry, with which they doubt- 
less connect, and which will be described in detail in a forthcoming report 
on the Ten-Mile district. 


This rock, which occurs only in the immediate vicinity of Leadville, is 
in its typical form apparently a decomposed Lincoln or Eagle River Por- 
phyry. It has the same mineral composition and frequently the large ortho- 
clase crystals that the former has, and can be traced as a continuous sheet 
through transition forms into the typical variety of the latter. It is almost 
invariably decomposed, and on or near the surface is generally a greenish- 
gray rock, showing numerous crystals in a prominent earthy-looking ground- 
mass; in the mines it is usually found bleached and often reduced to a 
white pasty mass in which the outlines of former crystalline constituents 
are but faintly traceable. It is of importance in connection with the ore 
deposits, as where it has crossed the Blue Limestone it has often played the 
same role with regard to them as the White Porphyry. 

As distinguished from the Lincoln Porphyry the microscope detects 
traces of former hornblende in the rock and finds glass inclusions in the 
quartz and numerous fluid inclusions in the feldspar. 

Occurrence. The main sheet of Gray Porphyry, the only body which is 
distinguished by a distinct color on the Leadville map, occurs above the 
main sheet of White Porphyry in the northern half of the area shown on 
that map, and extends beyond it to Mount Zion. Other bodies which 
belong without question to this variety, as well as those which are more 
doubtful, have, for reasons to be given below, been included under the color 
of "Other porphyries" on this map. The most important of these is a sheet 
occurring in the Blue Limestone, cutting transversely upwards from its base 
to the overlying White Porphyry. Among those which are doubtful are 


the Printer Boy and Josephine Porphyries, which occur the one on Printer 
Boy, the other on Long and Derry hill. Among rocks so thoroughly 
decomposed as are those in the immediate vicinity of the ore bodies it is 
often impossible to assign an occurrence with absolute certainty to a dis- 
tinct type; the miner can, however, in most cases distinguish these porphyries 
from the White Porphyry by the outlines of former crystals which the slight 
stain of iron oxide caused by their decomposition leaves. 


This rock in the hand specimen has the same general appearance as 
the variety of Lincoln Porphyry which has no large crystals. It is a dark- 
gray, granular, rather even-grained rock, in which the groundmass is decid- 
edly subordinate, and contains quartz, two feldspars, biotite, and horn- 
blende. It is distinguished from the former rock by carrying a much larger 
proportion of plagioclase feldspar, and hornblende as well as biotite. The 
microscope discloses the usual accessory minerals, with allanite and pyrite, 
and shows that the groundmass is holocrystalline and contains no glassy 
material. In the large masses of the higher mountain region it is usually a 
fresh-looking rock, but in mine workings and under a covering of soil and 
gravel capable of holding water it is usually much decomposed and bleached 
to a light-green, almost homogeneous-looking rock, with much epidote. The 
processes of decomposition in this rock, which are exceptionally interesting, 
are explained at length in Appendix A. 

Occurrence. The main laccolitic body of Sacramento Porphyry is found 
under Gemini Peaks, between the heads of Big and Little Sacramento 
gulches. A fine cliff section of the* body is also found on the face of Mount 
Evans towards Evans Amphitheater. It reaches a thickness of over a 
thousand feet in this i*egion. Its main sheet occurs above the White Por- 
phyry, or, when this is wanting, with an interposition of Weber Shales 
between it and the Blue Limestone. East of the London fault it rests 
directly on the Blue Limestone, and in the neighborhood of the Sacramento 
mine it plays the same role with regard to the ore deposits that the White 
and Lincoln porphyries do at other points. It also forms sheets higher up 
in the Weber Grits and less frequently in the lower Paleozoic strata In 



a broad, general way it may be said that on the eastern slope of the range 
Lincoln Porphyry extends from the northern edge of the map to Mosquito 
gulch, Sacramento Porphyry from Mosquito gulch to the ridge south of 
Little Sacramento gulch, and White Porphyry from there south to the limits 
of the map. The only point observed which showed evidence of a feeding 
channel from below was at the head of Little Sacramento gulch. 


This rock, though an extremely important element in the geology of 
the immediate vicinity of Leadville, does not occur outside that region and, 
like most of the eruptive rocks in the vicinity of the great ore concentra- 
tions, is in such a universally decomposed condition that its original constitu- 
ents cannot be definitely determined. It is generally of a white color, with 
grayish-green or pinkish tints, comparatively fine grained, and with no 
traces of large crystals. In it can be distinguished small grains of white 
feldspar, quartz, biotite which is generally altered to a chloritic substance, 
and pyrite. The last ingredient, from which it derives its name, is found 
abundantly scattered through the rock in crystals, often so fine as to be 
undistinguishable by the naked eye. They occur at times within the crystals 
of quartz and biotite, and are hence supposed to be an original constituent 
of the rock. They are frequently concentrated along cleavage planes, 
sometimes associated with finely disseminated crystals of galena. Pyritif- 
erous Porphyry is readily distinguished from the White Porphyry by its 
crystalline constituents. It differs from the Sacramento and Gray Porphy- 
ries by a relatively small amount of plagioclase feldspar and from the 
former by the absence of hornblende Its most strikingly distinctive feat- 
ure is the amount of pyrites which it contains, which is estimated to con- 
stitute, on the average, 4 per cent, of its mass. The only further constitu- 
ents disclosed by the microscope are minute crystals of zircon. Fluid but 
no glass inclusions are found. 

Occurrence. The Pyritiferous Porphyry,. as stated above, is confined to 
the area of the Leadville map, and is at present principally developed on 
Breece hill and the slopes of Ball Mountain. Its original extent previous 
to erosion was probably much greater than at present. It is a stratigraph- 


ical replacer of the Gray Porphyry on the north and of the Sacramento 
Porphyry on the east, occurring mainly above the Blue Limestone, but with 
either White Porphyry or Weber Shales interposed between it and that hori- 
zon. In California gulch it is also found at lower horizons, but apparently 
cutting across them upwards. 


This porphyry, a light-gray, fine-grained rock occurring exclusively 
in the form of dikes, is formed of quartz, two feldspars, and biotite. The 
quartz is very prominent, in clear, irregular grains; orthoclase feldspar 
is predominant over plagioclase; biotite occurs in small leaves and is not 
abundant. The occurrence of macroscopical apatite in glistening hexag- 
onal prisms is a noticeable feature of the rock. The microscope discloses 
a remarkable association of small ore grains (ilmenite, pyrite, specular hem- 
atite, and magnetite), together with zircon. 

Occurrence. The type rock was only observed in dikes in the Archean, 
viz, in the North Mosquito Amphitheater, on the north face of Mount Lin- 
coln, and in Cameron Amphitheater where it extends from the Archean up 
into the Paleozoic. 


This is a fine-grained, almost compact rock, of light-green color, result- 
ing from the chloritic decomposition of its original constituents, which renders 
their identification difficult. Quartz, two feldspars, biotite, and hornblende 
have been identified; but the relative proportions of orthoclase and plagio- 
clase are not readily apparent. Muscovite and calcite are decomposition 
products of the feldspars. The groundmass is often so subordinate that 
the rock seems macrocrystalline. 

occurrence. It is found as interstratified sheets on lower Loveland hill 
near the Fanny Barrett claim and in Cambrian quartzite on the north side 
of Mosquito gulch; also, as a dike running north across the Paleozoic beds 
from the lower edge of Bross Amphitheater. 


This rock forms important intrusive sheets on the mountain mass of 
Silverheels outside of the limits of the Mosquito map ; it has not been so 


carefully studied as the other varieties. It is an extremely fine-grained, 
greenish-gray rock, which in the hand specimen is characterized by fine 
needles of what is apparently decomposed hornblende. It carries quartz in 
small amount, two feldspars whose relative proportions are not readily 
apparent, with hornblende and biotite. These constituents are so very 
small as not to be readily distinguished. The microscope discloses the usual 
accessory minerals, including allanite and pyrite. The groundmass is 
holocrystalline and contains no glass. A porphyritic rock found on a south- 
ern spur of Mount Silverheels, at the forks of Crooked Creek, although of 
much coarser grain and more distinctly porphyritic habit, has essentially 
the same elements as the Silverheels Porphyry. 


Only three occurrences of granular plagioclastic rocks were found in 
the region, each of which was in the form of a dike cutting through the 
Archean in Buckskin gulch. The rock of each of these occurrences repre- 
sents a distinct variety of the type. 

Hornblende diorite. The normal diorite, which forms a broad dike cross- 
ing the head of the gulch, is a fine-grained, gray rock, in which the prom- 
inent constituents are plagioclase feldspar and hornblende, while a little 
quartz, brown biotite, yellow titanite, and dark ore grains can be detected 
by the naked eye. The microscope discloses also zircon and apatite, with 
chlorite and epidote as alteration products of the hornblende and biotite, 
and muscovite formed from orthoclase. A similar rock i's found in French 
gulch, on the west side of the range 

Quartz-mica diorite. This rock occurs on the south side of Buckskin gulch, 
opposite the Red Amphitheater. It is a dark, even-grained rock, in which 
quartz and feldspar are more prominent than the small irregular leaves of 
biotite; hornblende is wanting. The microscope shows zircon, magnetite, 
apatite, biotite, plagioclase, orthoclase, and quartz as original constituents. 

Augitic diorite. This rock, which is darker and finer grained than either 
of the preceding, occurs in the Red Amphitheater, cutting up through the 
Archean into the base of the Cambrian. In the hand specimen only horn- 
blende, biotite, plagioclase, and a little quartz can be distinguished, tmt the 



' v> 

- - ' 

Hornblende Poxpnyrites 


microscope detects also augite, orthoclase, zircon, titanite, magnetite, hema- 
tite, and apatite. 


As compared with the quartz-porphyries, the type rocks of this class 
are distinguished at first glance by a great predominance of basic silicates 
(hornblende or biotite), by a comparative rareness of quartz, and by their 
rather younger field habit, as shown by the marked conchoidal fracture 
and generally fresher appearance. For the latter reason it was at first 
thought in the field that they might possibly be of Tertiary age, but the 
fact that they are folded and faulted with the inclosing Paleozoic rocks, as 
well as their internal structure, proves them to be, like the quartz porphy- 
ries, of Secondary age. In their manner of occurrence they are also distinct 
from the latter rocks, in that they do not form large bodies, neither dikes 
nor intrusive sheets being as a rule over twenty feet in thickness. The 
former often occur in the form of interrupted dikes; the latter, on the 
other hand, while occasionally crossing from bed to bed, have a most 
remarkable extent in one general horizon as compared with the thickness 
of the sheet. Although subordinate in amount to the quartz porphyries, 
these rocks occur with so many variations of internal structure and compo- 
sition that they afford a complete series, including almost all the possible 
varieties of the type, and a complete description and classification made by 
Mr. Cross from a lithological point of view will be found in Appendix A. 
Only the general features of the rocks will therefore be given here. 

The typical rock, both in composition and manner of occurrence, may 
be taken as that which occurs interbedded in the Paleozoic beds along the 
cliff sections on either side' of Mosquito gulch. A photograph of a hand 
specimen of this rock is reproduced in Plate VII, Fig. 2, which gives some 
idea of its general appearance; it is a rather dark greenish-gray rock, with 
dark weathered surface and clean conchoidal fracture. The most promi- 
nent macroscopical constituents are well defined prisms of dark hornblende 
and small, white, opaque crystals of plagioclase. The microscope detects 
some biotite both among the porphyritic constituents and in the ground- 
mass, and both orthoclase and quartz in the groundmass No glass and but 
few fluid inclusions are found. 


Occurrence. The manner of occurrence of this rock in the region above 
mentioned is quite remarkable. It has been traced in practical continuity 
over an area of some four square miles, and probably has a much wider 
extent. It is regularly interbedded and rarely over twenty feet in thick- 
ness. It is easily traceable from a distance on the cliff walls, as a dark 
band between the lighter-colored sedimentary strata, and, while it appar- 
ently follows rigorously the same horizon, it is found, on close examina- 
tion, to cross from bed to bed at different points, so that its range in this 
area is actually from the upper part of the Cambrian to the top of the Silu- 
rian. The manner in which it crosses the beds is shown in Plates XIII 
and XIV. It also occurs at various other points in narrow dikes in the 

This rock forms Type V of Division 13 of Mr. Cross's classification, this 
division being that in which the hornblende and biotite are found both in 
the groundmass and as porphyritic constituents. His Division A includes 
rocks in which these basic minerals are entirely wanting in the groundmass, 
and which, in consequence, are of much lighter color than either of the 
other divisions. The rocks of his Division C, on the other hand, in which 
the hornblende and biotite are found only in the groundmass, are generally 
of darker color, and the arrangement of these minerals around the larger 
porphyritic crystals often shows a fluidal structure. 

Included fragments of pebbles of Archean rocks are more frequent in 
these than in any other eruptive rocks of the region, and in Plate VII, Fig. 1, 
is shown a specimen of a rock of Division A, from a remarkable dike in the 
Arkansas Amphitheater, in which the included fragments are large rounded 
crystals of orthoclase, whose presence in such form it has not yet been pos- 
sible to account for. 


The Tertiary eruptives found in this region consist of rhyolites and one 
occurrence of quartziferous trachyte within the limits of the Mosquito map, 
and of an interesting occurrence of andesite just south of those limits. The 
quartziferous trachyte being a small body, and of no great importance a 
bearing on the subject-matter of this report, has not been designated by a 
special color, but is included on the map under the rhyolite color. The 


eruption of these rocks had apparently no influence on the ore deposition 
of the region, since that, as well as can be determined, was pre-Tertiary, 
and no ore bodies have been found in connection with these rocks. Their 
interest is therefore chiefly lithological. 


The most important body, both in mass and in lithological interest, is that 
of Chalk Mountain, on the northern edge of the map, which, as the name 
of the mountain indicates, is prominent on account of its dazzling white color. 
It is a very crystalline rock, in which the groundmass is so subordinate as 
to appear in the hand specimen entirely wanting ; it corresponds, therefore, 
to the generally accepted definition of Nevadite. Its prominent constitu- 
ents are sanidine, generally in large crystals and having a peculiar satiny 
luster, and smoky quartz. The microscope also detects some plagioclase, a 
little biotite, with magnetite, apatite, and zircon in relatively small propor- 
tion as compared with the quartz porphyries. The quartzes contain fluid 
inclusions. A careful study of this rock by Mr. Cross has developed the 
fact that the peculiar luster of these feldspars is due to an actual parting, 
analogous to cleavage, which has already been determined as that which 
gives the blue color observed in the feldspar of many rocks, notably labra- 
dorite and some rhyolites. He also found crystals of topaz in some of the 
druses of this rock, the first instance, so far as known, in which this mineral 
has been found in Tertiary rocks. On Plate VIII is the reproduction of a 
photograph of a hand specimen of this rock, in which the smoky quartz 
grains appear black; above this are two microsections which show the sim- 
ilar granular structure of this rock and of White Porphyry. 1 

The next important body of rhyolite is that at the west base of Bart- 
lett Mountain, at the head of McNulty gulch, a tributary of the Ten-Mile 
Creek; it here cuts across porphyrite and quartz porphyry. This rock, 
though generally light colored, is not as white as the Chalk Mountain rock, 
nor is it so decidedly of the Nevadite type, the groundmass being often quite 
prominent. It contains glassy feldspars, quartz, and biotite. In darker 

1 In some of the plates, by an error iu proof-reading, the title White Porphyry, which belongs to 
the left-hand section, has been placed below the right-hand section and vice versa. The reader will 
bear in mind that the section containing the large crystal is Nevadite. 


portions of the rock biotite is quite abundant and some hornblende appears. 
The microscope shows glass, but no fluid, inclusions in both quartz and 
feldspar. The groundmass is cryptocrystalline. In general habit it is 
more like the recent volcanics than the Chalk Mountain rock, and yet, in 
some parts, it is with difficulty distinguished from a quartz porphyry. 

A third important body of rhyolite is that which forms Black hill, at the 
southeast extremity of the map. This is a light, often rather pinkish colored 
rock, of fresh habit and conchoidal fracture. It carries macroscopically 
two feldspars, smoky quartz, and some biotite. The microscope shows the 
groundmass to be granular, and that fluid inclusions occur in both quartz 
and feldspar and glass inclusions in the quartz. From the hand specimen 
alone the rock would be difficult to distinguish from an earlier quartz por- 
phyry, but the manner of its occurrence and its relations to the surround- 
ing rocks leave little doubt that it must be of Tertiary age. 

On the west slope of Empire hill a f-ne-grained, nearly white rock oc- 
curs below the White Limestone, which is distinctly orthoclastic and con- 
tains quartz and biotite. The fact that the quartz contains glass and no 
fluid inclusions points to a Tertiary age, but the occurrence has not been 
very carefully studied. A similar rock with larger crystals was found in a 
brecciated material from the Eureka shaft, in Stray-Horse gulch, which it 
has not yet been possible to account for. 

Trachyte. At the head of Union gulch are small irregular bodies, in 
granite and White Limestone, of fine-grained, dark-gray rock, full of brown 
biotite, with small glassy feldspars and some rounded yellowish quartz grains. 
The microscope shows hornblende and about equal portions of orthoclase 
and plagioclase. The groundmass is microfelsitic and has a fluidal structure. 
The quartz grains seem rounded and worn, and are confined to macroscopic 
individuals, for which reason they are regarded as accidental rather than 
normal constituents, and as the rock contains only 61.22 per cent, silica it 
is considered a trachyte rather than a rhyolite. 


The Buffalo Peaks form a double-pointed mountain mass, rising about 
a thousand feet above the main crest of the Mosquito Range, some ten miles 




White Porphyry 


fromChalk Mt 

inting Co. 211 rnt 


south of Weston's Pass. They consist of a normal hornblende-andesite, 
which is the cap rock, with a black vitreous rock which was at first consid- 
ered an augite-andesite, and a great development of tufaceous and breccia 
beds. A careful study of the darker rocks led Mr. Cross to the conclusion 
that their characteristic mineral was hypersthene, and to the establishment 
of hypersthene-andesite as a normal pyroxenic variety of this class. These 
rocks are described briefly in Appendix A, and more fully in No. 1 of the 
Bulletins of the United States Geological Survey. 



introductory. The following pages present a detailed description of the 
area included in the Mosquito map, summarized from field notes made 
during the summer of 1880. They contain the facts upon which have been 
founded the general conclusions drawn elsewhere with regard to the geol- 
ogy of this region, and therefore include many details that may not inter- 
est the general reader, but which will be of use to those who wish to use 
the maps on the ground or who desire to investigate critically the correct- 
ness of the generalizations. In preparing them it has been the aim of the 
writer to condense the description as far as could be done without omitting 
any essential observations. Circumstances made the time of field work 
extremely limited, and the detail in which it was possible to examine differ- 
ent parts of the region was necessarily unequal. The prime object of the 
work was to gather all information which might have bearing upon the 
origin and manner of formation of the ore deposits of the Leadville region. 
In the prosecution of this object much information of interest in other direc- 
tions has been collected, and many lines of investigation have suggested 
themselves which it would have been a pleasure to pursue further had time 
permitted. That such material be found incomplete is to be attributed, 
therefore, to a want of opportunity rather than of scientific zeal. 

In the following description the region has been treated in the general 
topographical order in which it was examined; that is, following the east- 
ern slopes of the range from the northern edge of the map southward to 
Weston's pass, and then along the west side in the inverse direction. Both 
geological and topographical structures lend themselves to this method of 


treatment, and permit four general divisions of the area: 1. The northeast- 
ern, including the Mount Lincoln massive, which, as shown in Plate IX, 
stands out quite by itself. 2. The middle- eastern region, or from Buckskin 
to Horseshoe gulch, inclusive. 3. The southern, including both sides of 
the range south of the line of Horseshoe and Empire gulches. 4. The 
northwestern division, including the area on the west side of the range north 
of the line of the Leadville map ; the middle area, which comes within the 
limits of this map, being described in a separate chapter. Each of these 
four divisions presents a general type of geological structure peculiar to 

The numbers after rock descriptions are the catalogue numbers of the 
specimens in the Leadville collection of the United States Geological Sur- 

Surface features. The whole region treated of in this report may be divided 
as regards its general supeificial characteristics into three belts or zones: 
(1) The bare summits and high ridges above timber-line; (2) the belt of 
forest growth covering the mountain slopes below timber-line ; (3) the open 
grass-grown and treeless valleys. 

The elevation of timber-line can only be given in a most general way 
as the average height at which tree-growth stops on the spurs where sur- 
face conditions are favorable.' The bare glacial amphitheaters in the in- 
terior of the range and the almost perpendicular walls of the canons present 
conditions unfavorable to tree-growth even at points below the timber-line, 
in spite of which the line is often well marked. Below an average elevation 
of 11,700 feet the flanks of the mountains are. covered with coniferous trees 
of the more hardy Alpine varieties, such as the Douglas fir and Engelman 
spruce, which in favorable situations often form a dense forest by no means 
easy to traverse, owing to the abundance of dead and fallen trunks, relics 
of former forest fires. The lower limit of tree growth is even more sharply 
defined; not, however, by its elevation above sea-level, but by the change 
of surface slope to the low angle which characterizes the valleys. Whether 
it be the bottom of a little mountain stream, a hundred feet wide, or the 
broad expanse of the South Park, almost as many miles in extent, the down- 
ward spread of forest growth is arrested with equal suddenness, provided 


only there be a sufficient thickness of loose detrital material, whether gravel 
or alluvial soil, accumulated over the hard rock surface. Along the alluvial 
bottoms of the streams, it is true, there is often a fringe of willow, alder, or 
cottonwoud ; but the sturdy pine, although delighting to face the mountain 
blasts on bare inaccessible precipices, seems afraid t/i trust himself where 
he cannot thrust his roots down to a base of firm rock, or around bowlders 
large enough to act as a counterpoise to the shaft he exposes to the force of 
the wind. 

The high mountain region, the forest region, and the valley region 
represent fairly three degrees of comparative difficulty in reading the 
geological story. In the former, except where covered by talus slopes at 
the foot of great cliffs, the rock surfaces are all laid bare and the geological 
structure is an open book, only needing an understanding and careful 
observer to be read correctly. In the forest region there is more or less 
accumulation of soil and decaying vegetable matter, and rock outcrops are 
often rare and widely spaced. The record has many gaps which time and 
care are not always sufficient to fill without resorting to hypothesis or 
analogy. In the larger valleys, however, whose surfaces are covered to 
unknown depths by gravel and soil, no outcrops are visible, and induction 
or analogy are the geologist's only resources for determining the structure 
of the underlying rock formations. 

Glacial formations. In the Arkansas Valley, as already noted, there is dis- 
tinct evidence of the existence of a glacial lake, and the Arkansas Lake 
beds, composed of stratified sands, marls, and conglomerates, have been 
actually exposed in a thickness of several hundred feet. In the South Park, 
on the other hand, no such stratified deposits have been observed, nor is the 
topography such as to suggest the possibility of a local lake of any great 
extent having been formed there during the Glacial period. While the 
existence of such a lake in the South Park is therefore considered improb- 
able, the fact that the exigencies of this work admitted the examination of 
only a small portion of its surface, immediately adjoining the Mosquito 
Range, does not justify a positive statement to this effect. 


Post-Giaciai formations. The Post-Glacial deposits of unstratified gravels 
are equally prominent, however, on both side* of the range. They result 
in great part from the redistribution of glacial moraines by the floods which 
accompanied the melting of the ice at the close of the Glacial period. In 
the Arkansas Valley they were spread out over the already existing Lake 
beds, and reach a relatively high level on the mountain spurs. In the 
western portion of the South Park they form the flood-plain of the larger 
valleys, which they filled up to a very considerable depth, as has been 
shown by excavations made at Alma and Fairplay in washing them for 
gold. Depths of 60 to 100 feet have here been proved of coarse gravel con- 
glomerate, entirely without stratification. These points are comparatively 
high up and near the source of supply, and it may be assumed that finer 
material of the same origin extends to equal if not to greater depths well 
out on the bottom lands of the park. Within these flood-plains the streams 
run in alluvial bottoms which widen as one descends and often open out 
into broad meadows, partially drained lake basins, where some natural ob- 
stacle has caused a partial damming up of the earlier streams. Of actual 
moraines no inconsiderable remnants still remain. They can be most clearly 
seen along the steep sides of the canon gorges through which the mountain 
streams debouch into the more open valleys, where they often form gravel 
ridges several hundred feet in height; and on the lower spurs beyond these 
canons their existence under the forest growth may often be surmised by 
their characteristic topography of irregular ridges inclosing rounded hollows 
without exterior drainage, as well as proved by shafts and tunnels made by 
the misapplied energies of prospectors. 

Archean exposures. To the lithologist no more favorable opportunity could 
be had for an exhaustive study of the older crystalline rocks which form 
the backbone of the Rocky Mountain system than that afforded by the 
exposures in the deep gorges and glacial amphitheaters of the interior of 
this range. The scope of this work did not admit, however, of any such 
exhaustive study, which would have required much more time than could 
have been devoted to the whole region. The utmost that could be done 
was to grasp the more salient characteristics of the series and to outline on 
the map such of the more important eruptive masses which intersect them 


as fell under observation, without pretending to present them in any deter- 
mined degree of completenss. The special study of the Archean rocks 
in the field was assigned to Assistant Whitman Cross, to whom also was al- 
lotted the duty of examining them microscopically, and the greater pail of 
the observations here recorded are derived from his notes. Granites and 
gneisses with accessory occurrences of amphibolite constitute, as already 
stated in Chapter III, the main components of the Archean in Mosquito 
Range. As seen from one of the commanding peaks of the range the 
most striking features of the rocks are the great irregular vein-like masses 
of white pegmatite, which form an infinitely intricate network on a 
background of darker gneiss. When examined more closely, however, 
the definite outline of these pegmatite bodies is no longer so apparent, and 
they are found to be intergrown in the surrounding rocks in a most intri- 
cate manner. It is only in the smaller veins, such as are shown in Plate 
IV, that their outlines can be definitely traced. Structure lines, as defined 
by relics of former stratification, are so seldom to be distinctly traced that 
no attempt has been made to co-ordinate the few facts observed into any 
general structural system. 

Of eruptive rocks in the form of dikes and intrusive masses of irregular 
shape an almost infinite variety, both in form and composition, is found. 
The dikes are generally narrow, being rarely over 50 feet in width, and of 
limited continuous length. Those shown on the map are only the more 
prominent of those actually observed, and it must be borne in mind that a 
great portion probably did not come under observation at all. 


Plane amphitheater. Like the Arkansas River, whose amphitheater adjoins 
this on the west, separated only by a single narrow, knife-like ridge, the 
Platte at its source flows first north and then bends round upon itself to 
take its main course in a diametrically opposite direction. A reason for 
this by no means .uncommon occurrence in the glaciated regions of the 
Rocky Mountains may be found in the fact that on the northern sides of 
the higher peaks are the greatest and most permanent accumulations of 

ice, to whose erosive action, not yet thoroughly studied, are doubtless 


Mi B-os* 

Brass Amphitheatre 

MA; ; iv 


S.K Kmmons. Geolotjisl- 



due the semicircular form and remarkable verticality of the upper walls of 
glacial amphitheaters or cirques. 

The main area of the Platte amphitheater lies directly west of Mount 
Lincoln, but a smaller northwest branch extends back of North Peak, hold- 
ing on its basin-shaped floor, which is about six hundred feet higher than 
the other, several pretty glacial lakes with characteristically emerald-tinted 
waters. The glacier formed by the confluence of the two immense neVe" 
masses that once filled these amphitheaters, which must have been about two 
thousand feet thick, flowed directly east, carving out a straight U-shaped 
valley in the crystalline rocks, whose general form remains essentially 
unchanged to the present day. 

On the upturned sedimentary beds which rest upon the Archean, how- 
ever, later erosion has acted more rapidly and irregularly, and at the little 
town of Montgomery the valley suddenly widens out into a broad, grassy 
bottom-land, with forest-covered hills sloping away more gently on either 
side. Immediately above Montgomery, as shown in Plate IX, 1 the present 
stream bends a little southward around a boss of Archean, composed chiefly 
of gneiss and amphibolite, penetrated by a fine-grained white granite, in 
which reticulated veins of white pegmatite stand out prominently. In the 
bottom of the valley, above this boss for a mile or more, extend glacier- 
worn hillocks (roches moutonne"es) of typical form, evenly rounded and 
scored by very distinctly-marked grooves and striae on the upper side, but 
breaking off unevenly on the lower side toward the stream. On either 
side of the gorge, above the talus slopes of broken rock masses at their foot, 
steep walls of Archean rocks rise about two thousand feet, with a thin 
capping of nearly horizontal Paleozoic strata at the very summit. The 
structure planes of the Archean, which are unusually distinct in the Platte 
gorge, stand nearly vertical, with a strike south- southeast. 

The eastern portion of the Archean mass seems mainly composed of 
gneiss and crystalline schists, granite occurring only in subordinate masses. 
The granite near Montgomery is of the gray, fine-grained type, suggestive 

'In this and the succeeding diagrammatic sketches, which are intended mainly to illustrate the 
geology of the various exposures shown, the letters on the outcrops are the same that are used on the 
geological maps to designate the different rock formations, i. e., a = Archean, 1> = Cambrian, c = Silu- 
rian, etc. 


rather of an eruptive origin, and contains relatively more mica and quartz 
than that found in Buckskin gulch. The gneiss is of the normal gray type, 
generally rich in quartz and biotite. Its feldspar occurs often in large 
Carlsbad twins. The microscope detects plagioclase, microcline, and mus- 
covite ; also, abundant fluid inclusions in the quartz, sometimes double and 
with salt cubes and moving bubbles. A schist found locally on the northern 
face of Mount Lincoln is of dark-green color and contains only biotite, 
muscovite, and tourmaline, with a little feldspar, which is scarcely visible, 
even under the microscope, and then appears in a stage of alteration into 
muscovite. The white pegmatite masses are specially prominent, as al- 
ready mentioned, on the faces of the spurs on either side of the gorge at 
Montgomery. Their color is due to the large proportion of white ortho- 
clase feldspar, which in the mass gives its tone to the quartz also, while the 
mica, generally muscovite, occurs in bunches of subordinate importance, 
growing between the crystals of the other constituents. 

The more prominent eruptive masses observed and which are indicated 
on the map are : 

1. Half a mile above Montgomery a dike of porphyry crosses the 
valley at right angles and can be traced for a considerable distance up either 
wall. It is a light-colored, felsitic-looking rock, in which only very small 
quartz grains and biotite leaves can be detected by the naked eye. It most 
nearly approaches the Mount Zion, or fresh variety of White Porphyry, 
and has a holocrystalliue structure as seen under the microscope. 

2. A mile above Montgomery is a wider dike of light-gray quartz- 
porphyry, whose distinguishing peculiarity lies in brilliant-green grains 
of epidote, which are scattered uniformly through the rock and which are, 
in part certainly, the result of the decomposition of biotite. Its ground- 
mass is also microcrystalline. 

3. A third dike is particularly noticeable for its peculiar form, changing 
half way up the cliff from a vertical to a horizontal sheet. This change of 
form is not unusual in dikes which extend up into the Paleozoic or regularly 
bedded rocks ; but this is the only instance in which it has been observed in 
the Archean. The rock belongs to the Mosquito Porphyry type, and is 
identical with that found (type No. 2) on the south face of Mount Lin- 


coin and at the head of the Cameron amphitheater. It is a light-gray, fine- 
grained rock, consisting of quartz, feldspar, and biotite crystals in a very 
scanty groundmass. The groundmass is a fine-grained mosaic of quartz, 
with some feldspar and muscovite, the latter resulting from decomposition 
of feldspar and probably in part also from fine biotite leaves, since this al- 
teration is visible in the larger individuals. 

4. Just west of this is a very irregular body of quartz-porphyry, not 
shown on the map. It occurs at the base of the cliffs and is very variable 
in form and thickness, branching out irregularly and continually changing 
its direction. It is a dull-green rock and belongs to the Green Porphyry 
type. It is rich in feldspar, with a few grains of quartz, and what is prob- 
ably a decomposition product of hornblende which gives the color to the 
rock. On the cleavage-planes are coatings of epidote. 

5. Still further up the valley, directly under the summit of Mount Lin- 
coln, is a dike of White Porphyry extending high up on the face of the 
cliff. It resembles closely the typical White or Leadville Porphyry, but is 
less decomposed. A few small crystals of quartz and feldspar are visible, 
also frequent light-green specks of partly decomposed biotite. It is almost 
identical with the similarly situated dike (dike No. 1) in Cameron amphi- 
theater, on the south face of Mount Lincoln, and with fragments found at 
the head of Buckskin amphitheater, for which this description will also 
apply. Its outer weathered surface is very white and homogeneous-looking; 
immediately under this is a dark zone, less than an inch in thickness, which 
apparently owes its color to the oxidation of some heavy metal originally 
contained in ore particles or in the biotite. It was impossible to obtain 
sufficient biotite for a chemical test to prove this assumption, which is 
founded on indications observed by the microscope. In the Cameron rock 
small crystals of pyrite could be detected, and in that from Buckskin a 
little galena also, whose decomposition would more directly account for the 
dirty-brown color alluded to. 

On the raised floor of the northwestern arm of the Platte amphitheater 
granite predominates among the Archean rocks. It is of the same variety 
as that found directly west in Bartlett Mountain and Clinton amphitheater, 
and has large and prominent crystals of feldspar disposed in regular order 



throughout the mass. The associated gneiss also contains large orV.ioclase 
crystals, often two inches in length and usually Carlsbad twins. On the 
surface of this floor was observed an interrupted dike of hornblende-por- 
phyrite, which is figured on the map ; also, small outcrops of other eruptive 
rocks, notably one of White Porphyry, whose outlines were not determined 
with sufficient accuracy to be there indicated. On the west wall of this 
amphitheater appears a dark line, which may probably be part of the same 
dike of porphyrite as is shown on the map to extend almost continuously 
along the west wall of the Arkansas amphitheater. Owing to their darker 
color and peculiar fracture in large masses, which is like that of a basalt or 
andesite, the porphyrite bodies can readily be distinguished at a consider- 
able distance. 

The North Peak ridge, which forms the northern wall of the Platte 
gorge, being lower than the corresponding spurs to the north and south, 
respectively, is composed almost entirely of Archean rocks, a proportion- 
ately smaller capping of Paleozoic strata being left on its crest. The actual 
outline of the remnant of Cambrian quartzite remaining on the ridge could 
only be determined with exactness by the expenditure of more time than it 
was possible t j devote to this point, and the line given on the map is that 
determined by observation of the apparent stratification lines from Mount 

Quandary Peak. On the Quandary Peak ridge, which lies just north of 
thelimits of the map, it is easily seen from a distance that a remnant of 
Lower Quartzite is left at the very summit of the peak, as shown in the 
sketch given in Plate X, which is taken from the summit of Mount Lincoln. 
The angle of inclination of these beds, which is 15, is less than that of a por- 
tion of the ridge, in consequence of which they have been eroded off the saddle 
immediately east of the peak, and are found again lower down on the spur. 
At the timber-line, which reaches only the eastern end of this spur, the dip 
steepens to 25. This line of steepened dip can be traced on all the prin- 
cipal eastern spurs of the range and corresponds very nearly with the mouth 
of the cafion gorges which have been cut in the Archean. It is often accom- 
panied by some apparent dislocation of the strata, the amount of which, 
owing to discordant dip angles, it was not easy to determine. For pros- 




pectors this line seems to have had especial attraction, and not without 
reason, since along it are the best exposures of the lower Paleozoic rocks, 
in which on this side of the range there has been a considerable concentra- 
tion of ore. 

The sketch given in Plate X is a view of the region adjoining the upper 
Blue River Valley, as seen from Mount Lincoln. To the left or west of 
this valley the hills are almost entirely Archean, with a few later sediment- 
ary beds resting against their eastern spurs. On Quandary Peak alone do 
they still extend up to the very summit. On the east of the valley are the 
hills surrounding the town of Breckinridge, made up of Mesozoic beds and 
numerous porphyry sheets, in which valuable ore deposits have been dis- 
covered and from the debris of which rich gold placers have been accumu- 
lated in the valleys. 

The Quandary Peak ridge is here described, although it does not come 
within the limits of the map, since it was the only point at which the search 
for fossils in the Cambrian quartzite was successful. On its eastern end, a 
short distance above timber-line and perhaps half a mile above the Monte 
Cristo mine, about fifteen feet of -greenish argillaceous slates, belonging to 
the upper part of this formation, are exposed by a prospector's tunnel which 
was run in on the north face of the spur. From these shales, after a dili- 
gent search, good impressions of the Potsdam species Dicellocephalus were 
obtained. Unfortunately the ground is too much covered by. soil and forest 
to afford a continuous section ; but, unless a fault intervenes, this shale bed 
should be below the quartzite and limestone in which the Monte Cristo 
deposit occurs, and not many feet from it. Lithologically it resembles the 
greenish shale beds observed in very many points throughout the region 
below the calcareous shales and sandy limestones of the upper portion of the 
Lower Quartzite series, but nowhere were any further traces of these fossils 

The exposures of the Cambrian or Lower Quartzite formation are never- 
theless those of the Paleozoic series which can be most clearly and con- 
tinuously traced, as they slope up in a U-shaped curve on either side of the 
valleys below the canon gorges of this portion of the range. In general 
the outcrops in the valley bottoms and along the lower slopes are concealed 


by surface accumulations, either talus slopes or alluvial soil. In the rela- 
tively wider valley of the Platte, however, about half a mile below the town 
of Montgomery, a moraine ridge which crosses the valley once dammed up 
a shallow lake basin, now a bit of meadow-land ; the present stream, which 
drains this basin, exposes as it cuts through this ridge the quartzites and 
shales of the Lower Quartzite formation and a considerable portion of the 
overlying White Limestone, striking N. 15 E. and dipping 20 to the east. 

Hoosier pass ridge. On the slopes of the Hoosier pass ridge, just above 
Montgomery, about one hundred feet of the Lower Quartzite are again ex- 
posed in section, with two parallel intrusive sheets of porphyrite, the one 10, 
the other 40 feet thick; the whole'dipping 25 to 30 east, with a strike to 
the west of north. This rock is the mica variety, having for its chief con- 
stituents a white plagioclase feldspar, with a much altered biotite and a few 
scattering quartz grains, in a dull-green groundnaass. 

The general line of contact of the Cambrian is traceable along the 
slope towards the crest of the North Peak ridge, but distinct outcrops are 
first found again at the saddle between Montgomery and the Blue River, 
over which a horse-trail leads. This saddle marks the outcrops of the Blue 
Limestone, which consist of a dark iron-stained dolomite, weathering black 
and carrying thin seams of barite. On the east of the saddle its limits are 
somewhat loosely defined by outcrops of blue shales, carrying casts of 
Zaphrentis and corals, which form a little knoll on the ridge, and may be 
assumed to belong to the shale member of the Weber series. On the west 
of the saddle, outcrops of the Blue and White Limestones extend to the 
steeper slopes of the North Peak ridge, where their limits are defined by a 
bed of green, fine-grained, silicious shale, impregnated with cubes of pyrite 
which at times forms beds a foot in thickness. Only the Lower Quartzite 
beds extend west of this on to the higher portion of the ridge. The work- 
ings of the now abandoned North Star mine on the first shoulder of the 
ridge have, as shown by the dump, passed through this quartzite into the 
schists of the Archean. 

On the northeast face of the ridge, overlooking the valley of the west 
fork of Blue River, is a small amphitheater with a little lake in its basin, 
which the topography of the map shews but imperfectly. It is entirely in 


the Archean, with the exception of a thin rim of Cambrian quartzite around 
its upper walls, and was probably carved out by a tributary of the main 
Blue River glacier, which descended the gorge from the back of Quandary 

A section was made from this saddle eastward across Hoosier pass 
to Hoosier Ridge, which connects the Silverheels massive with the group 
of hills to the north that constitute the eastern boundary of the Blue River 
Valley. This ridge also forms the divide between the waters of the Blue 
and Upper Platte Rivers and those of Tarryall Creek. 

No satisfactory measurements could be obtained of the thickness of 
the members of the Carboniferous group above the Blue Limestone, as was 
hoped : first, because the line followed did not cross the strata at right 
angles, but at times almost followe d the strike ; secondly, because of the 
great number of beds of porphyry included in the section, whose thick- 
ness could not be determined ; and, thirdly, because of the evidence of a 
syncline on Hoosier pass itself. Nevertheless, the data obtained are given 
here somewhat in detail, as it was one of the few opportunities offered during 
the investigation to follow continuously the ascending series of beds from 
the Blue Limestone up to the assumed top of the Carboniferous formation. 

From the saddle eastward to the grass-covered summit of the pass the 
outcrops may be assumed to indicate a thickness of about two thousand 
feet of beds. In this are included those of two prominent sheets of porphyry, 
which are apparently interbedded. On the first hill east of the saddle is an 
outcrop of shales, containing indistinct casts of fossils, apparently Zaphrentis 
and corals, which probably form part of the Weber Shales. The other 
outcrops are of the characteristic gritty rocks of this series (either micaceous, 
quartzose schists or coarse white sandstone, rich in muscovite and often 
- passing into conglomerate) and one bed of black argillaceous shale, which all 
show a conformable dip to the east and north. The grass-grown glades 
which form the summit of the pass leave a gap 'about half a mile without 
outcrops. Towards the eastern side, and overlooking the head of Blue 
River, a prospect shaft on the Ready-Pay claim has cut a body of light- 
gray limestone, which is probably one of the thin beds of limestone found 
in the middle of the Weber Grits series. This limestone, as well as an 


outcrop of coarse white sandstone a little east of the shaft, has a dip of 30 
to the westward, with a strike of about N. 25 W. On the slope of the 
pass towards the Platte Valley the Dead-Broke tunnel discloses what is 
probably the same bed of limestone, with sandstones dipping in the same 
direction. A body of light-colored mica- porphy rite is also cut in the end 
of the tunnel. 

The existence of a synclinal fold, as proved by these western dips, is in 
complete accord with the evidence, obtained farther south along the flanks 
of the ridge, of a secondary roll or minor fold in the strata parallel to the 
great fold of the center of the range, and explains the great thickness of 
exposures of Weber Grits beds. That the fold may have been accompanied 
by faulting is possible, but, as already stated, no direct evidence of a fault 
was found. Perhaps, had time permitted, a careful exploration of the ravine 

. at the head of the Blue River and on the west face of Hoosier ridge might have 
afforded more definite proof. As it is, the geological outlines given on the 
map are generalized from observations made on the spur connecting it with 
Hoosier pass. The results of these observations are graphically shown in 
section A A, Atlas Sheet VIII, for the eastern end of which, beyond the 
Platte Valley, they furnished the data. The largest body of porphyry 
there shown, which forms the shoulder of the spur above Hoosier pass to 
the east, consists of typical Lincoln Porphyry (54). It contains the usual 
large pinkish crystals of feldspar, which in this rock, however, seem excep- 
tionally susceptible to alteration and, instead of being fresh and rather 
glassy in appearance, are opaque and often quite kaolinized. The micro- 
scope shows rather more plagioclase than in the type rock, which may be 
accidental. The quartz occurs in small, double-pointed, hexagonal pyramids 
showing also the development of the prism ; and on the crest of the spur, 
where, owing to the gentle slope and accumulation of soil, decomposition 
seems to have gone on most freely, the rock surface is covered with a coarse 
sand made up almost entirely of such quartz crystals, often with well 
defined angles and facets. 

The steep north slope of the spur, facing the basin-shaped head of 
an eastern tributary of the Platte, shows a cliff wall of this rock with 

characteristic cross-jointings and vertical cleavage, almost amounting to a 


columnar structure. The thickness of the body can be hardly less than 
five hundred feet, as roughly determined from the widrii of the outcrop on 
the spur. That it forms so regular a sheet as shown in the section is an 
assumption based only on analogy from other sheets of porphyry observed 
in the Silverheels massive It apparently has its greatest thickness at this 
point, and thins out to the south and east, and in this respect has something 
of the laccolite form; but there is no evidence of any sudden steepening in 
the dip of the adjoining strata. On the contrary, the sandstone beds imme- 
diately overlying it, as shown in the outcrops on the crest of the ridge, have 
a regular dip eastward of about 10. Only a few hundred feet of sand- 
- stones and sandy shales separate this from the next succeeding sheet of por-" 
phyry, which forms the cap of the first prominent shoulder about twelve 
hundred feet above the pass. This is a blue-gray rock, weathering yellow, 
of quite distinct habit, having a conchoidal fracture and a tendency to 
weather into sherdy fragments. It approaches the normal Silverheels Por- 
phyry, although coarser grained, showing few distinct crystalline ingre- 
dients when freshly fractured. On its weathered surface, however, fine 
needles of hornblende are easily distinguishable. 

Beyond another body of sandstone and shales, and a similar though 
not identical body of porphyry which caps a second shoulder, a body of 
argillaceous shales of green, red, and purple colors marks what is assumed 
as the base of the upper division of the Carboniferous group. From these 
to the main crest of Hoosier ridge are several outcrops of porphyry sheets 
and intervening gaps of shaly rocks ; among which a bed of dark-blue 
limestone, about a hundred feet in thickness, stands out prominently on ac- 
count of its black weathered surface, opposite the head of the north fork 
of Beaver Creek. From this were obtained the following Coal Measure 
fossils : 

Athyris subtilita. Bellerophon, (sp. 1). 

Productus cora. FenesteUa, (sp. ?). 

Pleurolomaria, (P. Valvatiformis f). And spines of an Archceoccidaris. 
Loxomena, (sp. t). 

On the crest of Hoosier ridge are the reddish sandstones which form 
the passage from the Upper Carboniferous formation into the overlying 


Trias, dipping 15 to 20 east and north. Two other beds of limestone at 
least are found in this formation, on the same line of strike southward along 
the western face of Silverheels and in the valley of Beaver Creek, and they 
may occur here in some of the numerous covered gaps in the section. 

Silverheels Massive. In order to complete the somewhat meager data 
obtained upon the upper member of the Carboniferous group on this 
side of the range, the observations made in the region west of the Platte 
Valley will be next recorded, comprising in this the eastern portion of 
Mount Silverheels and Beaver Ridge, with the included valley of Beaver 

In a general way the eastern half of Mount Silverheels may be said 
to be Mesozoic, in great part probably Triassic, while its western face be- 
longs to the Upper Coal Measures, and Beaver ridge to the Weber Grits. 
The included porphyry sheets in the former rocks have a more recent and 
trachytic appearance, like that found at the forks of Crooked Creek ; those 
in the second group being rather of the Silverheels type, and those in the 
Weber Grits either identical with or similar to the Lincoln Porphyry. The 
number of these porphyry sheets is probably very much greater than 
is shown on the map, which represents a generalized outline of the more 
important bodies, deduced from observation made along three transverse 
lines only in the area represented east of the Platte ; while in that portion 
of the mountain which lies east of the boundary of the map the porphyry 
bodies are, if anything, still more numerous. The swelling out of the strata, 
produced by the intrusion of such considerable masses of eruptive rock, is 
readily shown by the variations in the strike and dip. The steep north wall 
of Silverheels, as seen from the summit of Hoosier pass for instance, shows 
a fan-like arrangement of the easterly-dipping strata, which open out as it 
were to the west. In other words, the section shows strata on the west foot 
of the mountain, towards Beaver Creek Valley, dipping only 10 east; at 
the summit of the peak the dip has increased to 17, while at the eastern 
extremity it is 22, 25, and even 35. The divergence in strike produced 
by the bowing-out of the strata is less evident on the map, owing to the 
fact that at the point of greatest divergence the great elevation of Silver- 
heels above the surrounding valleys brings the outcrops, as projected on a; 


map, so much farther west. A rough calculation of the difference in thick- 
ness of given east and west sections, taking the one on a line passing 
through Fairplay, the other through the summit of Silverheels, would give 
an increase in thickness in the latter case of 3,000 feet, which may be 
assumed as the aggregate mass of the intruded porphyry bodies at the latter 
point, since on the line through Fairplay they have very largely disappeared 
by thinning out. 

On a line eastward from Platte Valley to the summit of Silverheels the 
succession of rocks is as follows : Beaver Ridge, immediately adjoining the 
Platte Valley, whose steep slopes are covered with a thick forest growth 
which impedes observation, consists of the coarse grits of the Weber for- 
mation, with two principal and probably some minor bodies of Lincoln 
Porphyry. The valley of Beaver Creek, a straight depression in the line 
of strike, is apparently cut out of the softer shaly members at the top of 
this formation. From its bottom up the steep face of Silverheels are many 
porphyry bodies, whose de"bris often so obscures the outcrops that no con- 
tinuous section can be obtained. In this extent five sheets of porphyry and 
one bed of gray limestone were observed ; these alternate with shales and 
micaceous sandstones, which pass at the summit of the peak into conglom- 
erates. A considerable number of these conglomerates outcrop on the 
ridge running eastward from the summit, alternating with purple and 
green shales and with sheets of porphyry, of which no less than eight were 
counted. The conglomerates contain an unusual number of rounded and 
sub-angular fragments of the more resisting Archean rocks, together with 
the rounded pebbles of pinkish milky quartz which are common in all the 
sandstones of a coarser nature. Beyond them the brick-red sandstones of 
the Trias become the prevailing rock, their dip steepening on the east slope 
to 25 and 35. Along the west face of Silverheels the porphyry beds, 
which resist better the action of abrasion, can be traced in curving contours 
along the slopes, capping the more prominent shoulders of the spurs and 
disappearing from sight in the forests which clothe the lower spurs to .the 

The type of the Silverheels porphyry (89), which is found at the sum- 
mit, is a fine-grained rock of slightly greenish-gray color, having a con- 


choidal fracture, a sherdy habit, and a clear ring under the hammer. It is 
composed of feldspar, hornblende, and biotite, with a little quartz, and con- 
tains from 60 to 63 per cent, of silica. To the naked eye no groundmass 
is visible, although the crystalline ingredients are so minute (being gener- 
ally less than l mm in size) that they cannot readily be recognized. A com- 
mon variety (90) among the lower beds on the west and north is of coarser 
grain and more decidedly green color, due doubtless to the presence of 

The most southern of the three transverse lines above mentioned runs 
eastward from a little south of Alma, crosses several low forest-covered 
ridges separated by small valleys, and shows only detached outcrops sep- 
arated by frequent covered gaps. In this section only one body of por- 
phyry and three distinct horizons of dolomitic limestone were found. The 
beds, moreover, have a strike somewhat east of north and a dip of 25 or 
more to the eastward, instead of a strike to the west of north and dips of 10 
to 15, which prevail opposite the summit of Silverheels. The low ridge 
bordering the Platte Valley is covered on the west side nearly to its sum- 
mit by the lateral moraine of the Platte glacier, which must therefore at 
one time have filled the valley to a level about 400 feet above its present 
bottom. Lincoln Porphyry, a continuation of one of the bodies seen in 
Beaver Ridge to the north, is disclosed by prospect holes. Various deep- 
red sandstones are crossed, alternating with limestone and shales, but the 
characteristic brick red of the Trias is first found at 'Crooked Creek, to the 
east of Fairplay, in the forks of which is another important sheet of por- 
phyry, probably the porphyritic trachyte of the Hayden map. This is in- 
teresting as being different in appearance from any of the other porphyries 
observed in the region and resembling that found in a railroad cut through 
a Cretaceous ridge near Como. Nevertheless it does not possess the char- 
acteristics of a Tertiary rock, unless a slightly rough feel may be consid- 
ered such. It is of light-gray color and contains abundant porphyritically 
disseminated crystals, mostly of white opaque feldspar, in a subordinated 
groundmass. Two feldspars, hornblende, altered biotite, and quartz in large 
but infrequent grains form its macroscopical constituents. Microscopically 
the groundmass is seen to be evenly granular and the rock to be simply a 


porphyritic or coarser-grained modification of the Silverheels type, with no 
glass inclusions or other characteristics of Tertiary \olcanics 

Lincoln Massive. The Mount Lincoln massive, as is shown on the map and 
as may be seen in the sketch given in Plate IX (page 95), is divided by a deep 
glacial gorge, heading at the base of Mount Cameron, into two mountain 
masses : that of Mount Lincoln on the north and that of Mount Bross on 
the south. On the east face of either of these mountains are two smaller 
glacial amphitheaters, to which the names of their respective peaks have 
been given. The beds of each of these three gorges stand at a much higher 
level than the adjoining beds of the Platte and Buckskin gulches; and, if 
the glaciers which once filled them were ever directly connected with the 
main Platte glacier, later erosion has removed evidences of this fact. At 
all events, it is apparent that after the Glacial epoch, when the ice was 
gradually receding, these were separate glaciers or neVe" fields. This fact 
is more particularly manifest in the Lincoln amphitheater, in the middle 
of which stands a moraine ridge, outlined in the sketch above mentioned, 
which ends abruptly at the lower end of the amphitheater, about 700 feet 
above the level of Platte Valley. These amphitheaters have more signifi- 
cance geologically than their topographical importance would indicate, 
inasmuch as erosion, having once cut through the overlying and more 
resisting mantle of sedimentary beds, has carved deeply into the underly- 
ing Archean, leaving characteristic semicircular walls at their^ which 
afford most useful sections for studying the interior structure of the mount- 
ain mass. 

Mount Lincoln itself has three spurs stretching out to the eastward: a 
northeastern, an eastern, and a southeastern. Lincoln amphitheater is 
included between the two first. The surface of these spurs is covered by 
beds of the Paleozoic system, dipping eastward at an angle of 10 to 15. 
This is the average inclination of the beds over the main portion of the 
mountain mass; but, as already mentioned in the case of Quandary Peak, 
the dip becomes steeper on the extreme eastern flanks. In general, how- 
ever, the slope of the spurs themselves steepens for a short distance more 
rapidly than the dip, in consequence of which there is a belt of lower beds 
exposed along the foot of the steeper slopes. 


The eastern spur of Lincoln, a narrow straight ridge, being relatively 
much lower than the northeastern or southeastern spurs, is covered only by 
beds of the Cambrian formation, the White and Blue Limestones which 
still cap the other spurs having been removed by erosion. Section B B, 
Atlas Sheet VIII,' passes through this spur and shows its profile and geo- 
logical structure as well as can be expressed on so small a scale. In addi- 
tion to the normal eastern dip, the beds have also a decided inclination to 
the south, so that the spur presents a perpendicular wall on the north 
towards Lincoln amphitheater, with a shallow ravine on the south sepa- 
rating it from the southeastern spur, the slope of the spur in that direction 
corresponding nearly with the dip of the beds. This southern dip is the 
relic of a lateral fold or slight corrugation produced by the forces of con- 
traction acting in a northerly and southerly direction at right angles to the 
major force. The Lincoln amphitheater is thus shown to have been cut 
out of the axis of an anticlinal fold, and in the sedimentary beds still re- 
maining on the northeast spur a slight inclination to the northward can still 
be detected, showing that they formed the northern member of this subor- 
dinate fold. 

The Cambrian quartzites which form the mass of the spur are of the 
characteristic white saccharoidal variety, thinly and evenly bedded, and 
contain a slight development of white limestone, which has been occasion- 
ally observed elsewhere in this formation. At the eastern end of the spur 
is a cliff of quartzite, just above timber-line, below which the beds assume a 
steeper dip, so that the lower slopes are occupied by outcrops of succes- 
sively higher horizons. At the foot of this cliff are several prospect holes, 
following deposits of copper an'd iron pyrite near or in contact with a body 
of decomposed quartz-porphyry. A sheet of Lincoln Porphyry, which may 
be part of the same body, caps the spur above the cliff and is cut through 
by what seems to be a dike of porphyrite. The porphyrite contains both 
biotite and hornblende (the latter being, however, largely predominant) and 
is more decomposed than porphyrite rocks generally, both these minerals 

' By an*error in proof-reading, the line of this section, as given on the map in blue (Atlas Sheet 
VI), is partially wrong. It. should pass from the summit of Mount Cameron to that of Mount 
Lincoln, and from there down the eastern spur, whereas on the map it passes directly from Mount 
Cameron to the spur. 


being mostly altered to chlorite. Its groundmass is crystalline and con- 
tains a considerable development of calcite. Magnetite is also plentiful 
and has been frequently changed into hydrated oxide of iron. Muscovite 
is frequently present as an alteration product of plagioclase. The rock as 
usual contains many fragments of Archean, in this case of rnuscovite-gneiss. 
The Lincoln Porphyry is like the normal type, but contains few large 
feldspar crystals. Besides these is a more compact roc*k, apparently a 
contact product, which in general differs from either rock ; however, some 
specimens show its probable connection with the Lincoln Porphyry. Its 
biotite and hornblende are completely changed into chlorite and epidote. 
The groundmass is very fine and not resolvable into its elements. 

In ascending the regular slope of the ridge westward, as the dip of 
the formation is slightly steeper than this slope, successively lower beds of 
quartzite are crossed, and towards its upper end several interbedded sheets 
of porphyrite. These can be traced along the steep cliff wall overlooking 
Lincoln amphitheater, and are seen to follow the stratification lines for a 
considerable distance to the eastward and suddenly bend down into the 
underlying Archean, thus affording one of the few opportunities of observ- 
ing the change from a vertical dike into an interbedded mass. Owing to the 
contrast of the dark color of the porphyrite with the white including 
quartzite, these bodies can be distinguished from a great distance, and are 
distinctly visible from the opposite side of the Platte Valley, on the road 
which leads from Montgomery to the Hoosier pass. 

At its upper or western end, opposite the head of Lincoln amphithea- 
ter, this eastern spur merges into a basin-shaped valley with debris-covered 
slopes. On the east face of the northeastern spur, at the head of Lincoln 
amphitheater, a bare cliff wall affords a section of the lower sedimentary 
beds and included intrusive sheets, the whole mass much shattered and 
dislocated. Although time did not admit of the study of these cliff-sections 
in detail, as was done in the case of others which will be noticed later, the 
dark color of the intrqsive masses and fragments obtained from the debris 
show that they are largely of porphyrite, and therefore are probably parts 
of the sheet already noticed on the east spur. 


The upper surface of the northeastern and southeastern spurs of Lin- 
coln, respectively, is mainly formed of beds of Blue Limestone, which have 
been opened by innumerable prospect-holes and several considerable mines 
on either spur. On the steep cliff faces towards the Platte cafion and the 
Cameron amphitheater, respectively, the limits of this formation and those 
which underlie it can be distinctly traced. On the more rounded interior 
slopes debris of Lincoln Porphyry obscure very largely the actual rock 
surface. For this reason and also owing to the small scale of the map, the 
outlines of the formations there indicated are somewhat generalized. 

The sharp summit of Lincoln itself is made up of a mass of typical Lin- 
coln Porphyry, projecting boldly above the sedimentary beds and noticeable 
for its vertical cleavage planes, producing a columnar structure which is best 
seen on its steep south face. Lincoln Porphyry is also found for a consid- 
erable distance down the east spur, and with it are associated shales and 
grits belonging to the Weber Shale formation. The short, sharp ridge 
directly west of the summit of Lincoln, and between it and the saddle that 
separates Mount Lincoln from Mount Cameron, is also composed of a series 
of beds which evidently belong to this horizon. They dip somewhat 
sharply to the east and consist of greenish, yellowish, and reddish shales and 
of micaceous quartzites, with a bed of black shale near the top, comprising 
in all a thickness of about two hundred and forty feet. Below this is a bed 
of Lincoln Porphyry, evidently interstratified, while on the saddle itself are 
outcroppings of Blue Limestone. A deserted mine on this saddle, known 
as the Present Help, the highest mine probably in the United States, is ap- 
parently at or near the contact of the Blue Limestone with the overlying 
porphyry ; its workings had been abandoned and were inaccessible. The 
intense metamorphism shown in all the sedimentary beds near the summit 
of Lincoln and the columnar structure of its porphyry render it probable 
that the mass which forms the peak is directly above the channel through 
which this rock was erupted. There is evidence also that from this channel 
a sheet of the same rock was spread out over the surface of the Blue Lime- 
stone, wWch was probably the determining cause of the great concentration 
of mineral at this horizon. 


The typical Lincoln Porphyry, as found on the summit of Mount Lin- 
coln itself, is characterized by large orthoclase crystals, which sometimes 
reach two inches in length, of pinkish color, generally Carlsbad twins, and 
often so fresh and glassy in appearance as to remind one of the sanidine 
crystals of more recent rocks. There are five or six large crystals as a 
rule in an ordinary hand specimen. The smaller feldspars are white, and 
a large number show distinct striae. Quartz is very abundant and rela- 
tively large, in round grains, often of pinkish hue, and showing more or 
less plainly the faces of dihexahedral crystals. Biotite in darker or lighter 
green leaves, according to its condition of decomposition, is quite conspic- 
uous in the rock. A few specks of specular iron are sparingly scattered 
through the rock. The groundmass is light, green or pinkish and is quan- 
titatively quite subordinate to the crystalline element. Under the micro- 
scope it is seen to be fully crystalline. 

Such is the typical Lincoln Porphyry, which projects in lofty columns 
from the summit of the mountain in a sharp apex which overtops all the 
surrounding peaks. Owing to its exposed situation it attracts the storm 
clouds from all the regions around, and even in midsummer scarcely a day 
passes without a slight fall of snow or hail on the summit. The very 
topmost rocks show traces of discharges of the electric fluid in the forma- 
tion of fulgurite, which encircles the little holes it has bored into the rocks. 
Around the base of this summit mass of porphyry its contact with the sed- 
imentary rocks is obscured by debris, the few outcrops that are seen being 
composed of rocks so much altered that their original character cannot be 

Cameron amphitheater. On the steep south face of Lincoln, a sketch of 
which is shown in Plate XI, a careful study was made of the various erup- 
tive masses. The Lincoln Porphyry of the eastern edge of the summit is 
of a much darker color than the normal rock and contains few or none of 
the larger feldspar crystals. It is so much decomposed that only in the 
center of large blocks is the original grayish color preserved ; but the round 
quartz grains are distinct throughout. The Blue Limestone, -which here 
seems to have a brecciated structure, can be traced as a horizontal line 
across the face of the cliff, from the Present Help mine on the west to the 


Russia mine on the east, immediately below a bed of Lincoln Porphyry. 
Along the edge of the steep ravine which descends directly from the summit 
of Lincoln an irregular dike of porphyry crops out here and there, colored 
brilliant red and yellow on its surface, but so much decomposed that its 
original structure can no longer be determined. As shown in the sketch, it 
is only the Silurian (c) and Cambrian (b) strata which form continuous out- 
crops across the cliff face, and these are somewhat broken by transverse 
dikes of eruptive rock. Within the Cambrian quartzite is an intrusive sheet 
of Lincoln Porphyry, whose darker color contrasts strongly with the bleached 
weathered surfaces of the summit rock. The base line of the Cambrian, 
where it rests on the Archean, appears more irregular in the sketch than it 
is in nature, but it is evident that the Cambrian sea bottom was not so 
smooth here as it is shown to be in other cliff sections. 

In the ravine next east from that already mentioned is a dike of White 
Porphyry, which can be traced, as shown in the sketch, from the gneiss of 
the Archean across the Cambrian quartzites into the White Limestone. This 
is dike No. 1, whose rock has already been described under that which 
occurs on the north face of Lincoln. Its outline is extremely irregular, and 
its contact surfaces' with sedimentary rocks, which are distinctly visible, 
show none of the contact phenomena supposed to result from the heat of a 
fused mass. In its upper portion it is rounded, and curves over one of the 
heavier beds of White Limestone in an oval mass. On its east side, near its 
summit, the thinner beds of limestone are bent upwards, as if displaced at 
the time of its intrusion, and the lower shale beds of the White Limestone 
belt are more or less serpentinized. It also sends out offshoots a few inches 
wide through the natural joints of the sedimentary beds. About fifteen to 
twenty feet above the base of the Lower Quartzite it crosses an interbedded 
mass of porphyry of a dark-green color, which is here some thirty feet in 
thickness. This interbedded porphyry is thoroughly decomposed, the only 
crystals visible being rounded quartz grains, which resemble those of the 
Lincoln Porphyry. All its cleavage planes are covered by a dark-green 
coating of chloritic nature, and it is crossed by thin perpendicular fissures, 
from one to two inches in thickness, containing pyrites and having a bright- 
yellow weathered surface. A comparatively fresh specimen was obtained 


with some difficulty, which shows the characteristic large, pink, orthoclase 
feldspars of the Lincoln Porphyry. In this the green color is seen to be 
due to the alteration of biotite into a chloritic substance, which has been 
deposited on the surface of the smaller feldspars, so that they are scarcely 
distinguishable by the naked eye. Biotite is also no longer visible except 
under the "microscope. Pyrite can be distinguished throughout the rock by 
the naked eye. 

The Archean rocks (a) at the base of this section consist almost en- 
tirely of dark-gray gneiss. In this the White Porphyry dike can be traced 
but a short distance, as it is soon lost under the steep talus slopes at the 
foot of the cliff. 

A few hundred feet east of this dike (to the right in the sketch) a second 
dike (No 2) can be traced, though less distinctly, from the gneiss entirely 
across the Cambrian and Silurian formations, apparently terminating at the 
base of the Blue Limestone. It is much narrower and straighter than dike 
No. 1, and like that seems to have a northeast and southwest direction. Its 
rock is a light-colored, fine-grained, highly-crystalline porphyry, belonging 
to the type designated as Mosquito Porphyry, which has already been de- 
scribed. There is an outcrop of the sama rock in the Archean, on the west 
wall of the Cameron amphitheater directly under Mount Cameron, which 
may possibly be part of the same body, although the intermediate region 
is too much obscured by de'bris to trace any direct connection. 

Eastward of this cliff face the northern wall of the Cameron amphi- 
theater is much covered by de'bris for the distance of nearly a mile, in which 
extent, although the general dip of the sedimentary beds can be traced, no 
opportunity was presented for an examination of the intrusive bodies. 
Near the eastern end of the wall, however, is a second cliff section, which 
shows in a very instructive manner the position of the intrusive masses and 
dikes. It is graphically represented in Plate XII, which, like the preceding 
plate, is copied from sketches made on the spot by Prof. A. Lakes. The 
section was studied by Mr. Cross, from whose notes the following descrip- 
tion is largely taken. 

Here, as in the section just described, is an intrusive interbedded mass 
of porphyry in the Lower Quartzite (), only a few feet above its base, 



which is also crossed by a nearly vertical dike. This vertical dike, as may 
be seen on the left half of the sketch, can be traced from the Archean up 
to the base of the Blue Limestone. It is from fifteen to twenty feet wide at 
the bottom and branches at the top into five small arms, but does not spread 
out between the strata. Its rock is a White Porphyry, which differs from any 
of those observed elsewhere in carrying large orthoclase feldspars, sometimes 
an inch in length. They are Carlsbad twins, and have a pinkish tinge like 
those in the Lincoln Porphyry. Small rounded grains of quartz are also abun- 
dant, but no trace of hornblende or biotite could be seen, either by the naked 
eye or with the microscope. Under the microscope the feldspar is seen to be 
partly plagioclase, and in the quartz are many small fluid inclusions. The in- 
terbedded porphyry mass, like that on the south face of Lincoln, is prominent 
by its dark color; but on examination it is seen to consist of two distinct 
rocks, one of which seems to have pushed its way through the other after 
it had been already spread out between the beds. The later rock is a Lin- 
coln Porphyry, whose outlines can be distinguished from a little distance 
by its peculiarity of weathering, its fragments showing larger surfaces than 
that of the earlier rock. The earlier rock is of a light-green color, and 
shows, to the naked eye, scarcely any distinguishable crystals, feldspars 
being decomposed to a substance very like groundmass. Altered horn- 
blende, a few biotite leaves, and an occasional quartz grain can be distin- 
guished by the lens ; also, a few small specks of some metallic combination. 
Under the microscope the groundmass resolves itself into a fully crystal- 
line admixture of quartz, mica, and feldspar. Calcite is present in filmy 
particles and occasionally in grains. The larger quartz crystals contain 
fluid inclusions. The contact specimens of these two porphyries show a 
blending of the characters of the two in the tendency to the formation of 
large quartz and pink feldspar crystals in a base more like the older por- 
phyry. As shown in the sketch, the Lincoln Porphyry throughout the 
greater extent of the section is entirely included within the older mass. 
Towards the eastern end, however, it forms a distinct bed above the other, 
and each sends off a branch upward in a northeast direction across the strata, 
forming nearly parallel dikes which meet at the surface of the ridge. These 



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dikes are about fifteen feet in thickness each, while the combined beds have 
a thickness of from fifty to sixty feet. 

This intrusive sheet of Lincoln Porphyry at the Cambrian horizon, 
which seems continuous along the north wall of the Cameron amphitheater, 
was traced out to the end of the southeast spur of Lincoln ; and what is 
apparently the same bed was also observed lower down the slopes, in the 
more steeply-dipping members of the same formation. Outcrops of simi- 
larly situated bodies, as shown on the map, are also found on the south wall 
of the Cameron amphitheater and on either wall of the Bross amphitheater. 
Time did not permit of tracing any connection between these different out- 
crops ; and it seems doubtful whether any exists, inasmuch as for some un- 
known reason there seems to have been much less tendency to spread out. 
in extensive sheets at this horizon than at that above the Blue Limestone. 
Although this latter porphyry bed is only found to a limited extent above 
the Blue Limestone on Mount Lincoln, there is no doubt that it once covered 
that bed, forming a sheet comparable in extent to that of the White Por- 
phyry in the Leadville district. 

Cameron and Bross. The summit slopes of Mounts Cameron and Bross, 
except those on the cliff faces which are too steep to permit the lodgment 
of debris, are mainly covered by fragments of Lincoln Porphyry. Eruptive 
rocks under the action of atmospheric degradation split up into fragments 
whose shape and relatively small weight, as- compared with their superficial 
area, render them more susceptible to being moved by melting snow, so 
that on mountain sides they generally cover a surface disproportionately 
large as compared with their actual outcrops. This is eminently the case 
on Mount Bross. where angular fragments of porphyry often cover the 
surface to a depth of ten feet or more and the character of the underlying 
rock can often only be determined by actual excavation. 

The porphyry of the summit of Mount Cameron is remaukable for the 
unusual development of large orthoclase crystals, often more than two 
inches in length, which weather out from its surface. Associated with the 
much-weathered fragments of porphyry are various brown quartzitic sand- 
stones, which may represent a bed of the Weber Grits formation not yet 


eroded off the summit. No sufficient evidence was found, however, to 
justify its indication on the map. 

On Mount Bross the Lincoln Porphyry shows a still lighter color than 
that on Mount Lincoln, which seems due to the fact that the decomposed 
mica, instead of remaining as chlorite, has been entirely removed Frag- 
ments of shales and quartzitic sandstones of the Weber Grits formation are 
mingled with the porphyry debris of the upper slopes of Bross, and out- 
crops of these rocks are found on the ridge connecting it with Cameron, us 
well as to the south of its summit on the ridge overlooking Buckskin guhh- 
In the latter instance they stand at a much steeper angle than the lower 
series of Paleozoic beds, and give evidence of some local movement. In 
Section L, Atlas Sheet X, is shown the probable form of the Lincoln 
porphyry body on the summit of Mount Bross, as deduced from observed 
outcrops. It is very possible that, like that of Mount Lincoln, it stands over 
a channel of eruption, but the evidence of this was not considered strong 
enough to justify its being indicated on the plane of the section. 

On the north face of Mount Bross, towards Cameron amphitheater, 
the base line of the Paleozoic formations can be traced with tolerable dis- 
tinctness. Of dikes crossing the formation, like those on the face of Mount 
Lincoln opposite, there are doubtless many, but only one was actually 
traced, which is cut by the western workings of the Moose Mine. In the 
Archean below this mine is a prominent mass of light-gray granite. The 
workings are in the Blue Limestone, which is exposed on the east spur of the 
mountain between the Cameron and Bross amphitheaters, forming the sur- 
face of the spur, until cut off by its steeper slope, whose angle is greater 
than that of the dip of the beds. This bed is completely honeycombed by 
abandoned mine workings, but the underlying White Limestone here, as in 
the Leadville district, seems to have yielded little or no ore. At the foot 
of the spur, erosion has exposed the quartzite beds of the Cambrian, in 
which is a prominent dike of porphyry running from the edge of Bross 
amphitheater a little north of east, in the direction of the summit of Mount 
Silverheels. It was traced as far as the secondary ridge bordering the 
Platte Valley into the White Limestone, where it was lost in the forest. 


The rock (88) of which it is composed differs from any yet described. Its 
weathered surface is so white that at first glance it might be taken for the 
White or Leadville Porphyry. On a fresh fracture it has a light-green 
color and shows few macroscopical crystals. It has certain resemblances 
to porphyrite and also to the Silverheels Porphyry, but the microscope 
shows it to be identical with the quartz porphyry found on Loveland Hill 
and on the north wall of Mosquito Gulch, which has been described under 
the name of Green Porphyry. 

Bross amphitheater, like those of the other two peaks, lies nearly due 
east of the summit, but, owing to the steeper inclination of the Paleozoic 
beds which cap its walls, it has not been carved to so great a depth into 
the underlying Archean schists, whose outcrops are therefore of much less 
superficial extent. As in the others, the highest beds exposed in the cliff 
sections on its walls are those of the Blue Limestone. Shales, probably 
belonging to the Weber Shale formation, are disclosed in prospect holes 
along the road which curves round its head, and very possibly a consider- 
able portion of the area which has been given the color of porphyry on the 
map may prove by actual excavation to be underlaid by beds of this for- 
mation. The road which leads by the Dolly Varden and Moose mines; 
along the north face of Mount Bross and the west face of Mount Cameron, 
to the Present Help mine, on the south face of Mount Lincoln, is indicated 
on the sketch in Plate IX by a light double line, the location of the respect- 
ive mines being shown by the house outlines. The Dolly Varden mine, on 
the spur south of the amphitheater, finds its ore in the Blue Limestone adjoin- 
ing a dike of White Porphyry 40 feet in thickness, which crosses it at an 
angle of 60 with the horizon. Below the Dolly Varden mine the spur 
slopes more steeply than the beds, and at its base the Parting Quartzite of 
the Silurian is exposed. In the basin-shaped valley called Mineral Park, 
south of this spur, erosion must have exposed still lower beds than on the 
spur, and it is possible that the quartzite beds said to be exposed there may 
belong to the Cambrian. 

The ridge running south from Mount Bross, between Mineral Park and 
Buckskin gulch, is mainly covered by easterly- dipping beds of the Blue 


Limestone horizon. There are several bodies of Lincoln Porphyry, besides 
the main sheet near the summit, which are not shown on the map, as time 
did not admit a sufficiently detailed study to determine their outlines or 
whether they were remnants of this sheet or distinct bodies. The upper 
part of the Blue Limestone on this spur seems to have been particularly 
rich in black chert concretions, which now lie scattered over the surface of 
the ground, and from which Prof. Lakes obtained the following fossils: 

Spiriferina (sp. like 8. Spergenennis). 
Spirifera Rockymontana. 
Prodnctus costatus. 
Euomphalus (sp. f ). 

Athyris subtilita. 
Streptorynchm crassus. 
Pleurophorus oblongus. 

These were mainly collected in a slight depression of the ridge, where 
the overlying porphyry had been eroded off, and therefore must have 
come from the upper part of the horizon. 

The lower Paleozoic beds are exposed in section at various points 
along the steep western wall of this spur, which faces Buckskin gulch. 
They were examined at two points. At the extreme southern end of the 
spur, just above the town of Buckskin Joe, where the steeper eastern dip 
of the formation comes in, several ore bodies have been discovered, and the 
now abandoned mines (the Excelsior, in White Limestone, and the Cri- 
terion, in Lower Quartzite) were once worked. At the Criterion mine a 
thickness of 150 feet of quartzites was measured between the Archean and 
the first bed of White Limestone. The ore bodies are accumulated here 
along vertical planes, running northeast and southwest, which seems to be 
the direction of a dike of dark-green decomposed porphyrite, whose out- 
crops are found in the ravine below the mine, near the contact with the 
Archean. There is evidence also of a slight displacement along a plane 
running northeast and southwest, whose upthrow is to the west. At the 
Excelsior mine, which is about a quarter of a mile farther west, near the 
point of the cliff in the angle of the gulch, the ore bodies follow similar 
and nearly parallel planes. A section measured on the cliff near the mine 
gave the following thicknesses, in descending series : 



Blue Limestone, covering surface of spur ? 

a-, . < Parting Quartzite (exposed in prospect holes) t 

\ White Limestone, partly covered by debris, estimated . . 200 

Shales and sandy limestones 35 

Gray quartzite, impregnated with metallic mineral 20 

Massive white quart zite G 

Cambrian < Greenish quartzite, with calcareous layers 8 

White saccharoidal quartzite 10 

Greenish-white, compact, thin-bedded limestone 3 

i, White saccharoidal quartzite 55 


Archean ! 

The limestone bed in this section is of interest as being the only one 
examined from this region which was not a dolomite. It contained 25.48 
per cent, carbonate of lime, 4.03 carbonate of magnesia, with traces of chlo- 
rine, the residue being mainly silica. It has already been noted that the 
Cambrian beds in their upper part are often more or less calcareous, but 
generally resemble a sandstone on the surface, whereas this bed has the 
compact, even texture and clean fracture of a limestone. The strata at this 
point dip 15 to the east, with a strike a little east of north. 

Red amphitheater. Nearly under the summit of Mount Bross and high 
up on the east wall of Buckskin gulch is the Red amphitheater, a semi- 
circular recess in the cliff-wall nearly a thousand feet above the bed of the 
valley. The scale of the rnap does not permit an adequate expression of 
the form of this remarkable basin, which is rendered still more prominent 
by the brilliant red and yellow coloring of its walls. This color comes 
from a thin coating of ocherous clay, which covers the rock fragments of 
debris piles, and which contains, besides oxide of iron, traces of arsenic, 
antimony, and sulphur. The rock fragments thus coated are so much 
decomposed that it is seldom possible to determine their original character, 
and it would have taken much more time than was available to thoroughly 
decipher the geological history of this remarkable locality, which has evi- 
dently been the scene of long-continued metamorphic action, probably a 
sequence of the eruption of the igneous rocks now forming dikes and intru- 


sive sheets in the Archean and overlying Paleozoic beds. The results of the 
rnetamorphic action are shown, not only in the decomposition and coloring 
of the rocks above mentioned, but in the marbleizing of the limestones and 
the large development of serpentine within these limestones. 

The eruptive bodies developed here consist, besides the large body of 
Lincoln Porphyry near the summit of Mount Bross, first, of a considerable 
body of augite-bearing diorite (96), which cuts through the Archean from 
the valley below up into the bed of the amphitheater, and either spreads 
out along the base of, or extends into, the Cambrian beds under the talus 
slopes of debris ; secondly, of a dike of White Porphyry, crossing Silurian 
and Carboniferous limestones in a vertical direction ; thirdly, of several thin 
intrusive beds of green and much-altered quartz-porphyry, parallel with 
the stratification. It is only on the south side of the amphitheater that a 
continuous cliff-section of the Paleozoic beds is exposed, and here the top 
of the Blue Limestone and the base of the Cambrian are each covered by 
surface accumulations. One principal and several smaller faults can be 
distinguished on the cliffs, in each of which the upthrow is to the west, but 
the amount of displacement is only slight. The Colorado Springs mine is 
opened on this cliff, near the base of the Blue Limestone, from which rich 
ore in small quantities has been obtained. The following section was made, 
by means of a pocket level, on the cliff just south of the mine and near (he 
dike of White Porphyry above mentioned : 


! Black cherty limestone 50 

Blue-gray limestone 50 

Ligbt-blue limestone \ 

hite marbleized limestone I GO 

^ Light drab limestone witli serpentine ' 


White and greenish quartzite 40 

White limestone 10 

Light -bluish limestone 40 


Green porphyry, 20 feet. 

White limestone 10 

Blue-gray crystalline limestone ... 40 

. Light-colored limestone with serpentine 30 




\ Dark-green serpentine ......................... 10 

White limpid quartz ......................... 15 

Yellowish -greeu serpentine .................... 1 

Green porphyry, 4 feet. 

White quartzite ............................. 10 

Grecn 1'orphyry, 20 feet. 

Wliite quartzite ...................... . ........ 10 

Green porphyry, 5 feet. 

White qnartzite ............................... 40 

Intrusive mass, disturbing strata and disappearing 
under debris ................ ............... (?) 

Neither the top of the Blue Limestone nor the base of the Cambrian is 
reached in this section, and to the aggregate thickness given an unknown 
amount, probably in the neighborhood of 200 feet, should be added. At 
the head of the amphitheater above the Blue Limestone is a very thick body 
of Lincoln Porphyry, above which, on the summit of the ridge and sepa- 
rated by a low saddle from the main summit of Mount Bross, are intensely 
altered shales, frequently chloritic, belonging to the Weber Shale formation. 

The development of serpentine, which elsewhere seems confined to the 
"sandy limestones" of the upper part of the Cambrian, here extends, though 
on a minor scale of development, a short distance into the silicious beds 
below and up as far as the base of the Blue Limestone. The serpentines 
obtained from here are remarkably beautiful rocks, grading in color from a 
homogeneous yellow to a dark green, mixed with gray and having the 
general effect of a veined verd-antique, although more critical examination 
shows that the green and gray or yellow are a simple shading oft' and inter- 
growth. In some cases thin, vein-like sheets seem to cross the strata, though 
in general the development of serpentinous material is parallel to the strat- 
ification. Under the microscope they are seen to contain a very consider- 
able amount of calcite, an appearance which is confirmed by chemical 
analysis. The development of serpentine is apparent, in looking at the 
cliffs from a little distance, as a lenticular-shaped body, giving at first the 
impression that it causes an actual thickening of the beds; but the measure- 
ments given by the above section show that this is not the case, and the 
chemical examination, which is discussed in Chapter VI, shows that this 


mineral is the result of a change within the rocks themselves, and, proba- 
bly in great part, of the alteration of pyroxene and amphibole in the lime- 

Eastern foot-hiiu. The higher part of the Lincoln massive thus far 
described may be considered structurally to form part of the crest of an 
original great anticlinal fold, inasmuch as the average inclination of the 
beds is comparatively small. The wooded ridges between the foot of the 
steeper slope and the Platte Valley, which form a low shoulder to the Lin- 
coln massive, and where steeper dips prevail, would form the actual eastern 
member of the fold. On the ridge between Quartzville and Montgomery, 
for instance, the beds dip as steeply as 45. South of this a wider region 
is included between the outcrops of Blue Limestone and the Platte Valley, 
and, were the steep dips continued without interruption, an immense thick- 
ness of beds would be represented. There are reversed dips found, however, 
notably in the ravine below Quartzville and in Buckskin gorge above Alma, 
which give evidence of the existence of a secondary flexure parallel to the 
main fold, a sort of minor ripple following at the heels of the great breaker 
or wave which caused the main uplift of the range, such as is almost inva- 
riably found along lines of great plication. Another noticeable feature in 
the structure is a decided change of strike, which commences opposite the 
east spur of Mount Bross, or between the Bross and Cameron amphithea- 
ters. North of this line the average strike of the beds is north or a little 
east of north; south of it the strike bends more and more to the east of 
north; and on the southeast slopes of Bross the strata have a dip with the 

slope to the southeast. 

The outer wooded ridge above mentioned is composed of coarse sand- 
stones of the Weber Grits formation and of various intrusive bodies of por- 
phyry. Porphyry bodies similarly situated were observed on four distinct 
section lines followed across this ridge, but the assumption that they form 
part of a continuous body, as indicated on the map, is here, as in the case 
of those on the east side of the Platte Valley, not founded on the tracing of 
a continuous line of outcrops, as in the canon sections. They generally 
belong to the Lincoln Porphyry class. That found in the ravine above 
Dudley in considerable thickness has the round pink quartz grains, but wants 


the striking- large orthoclase crystals of the Lincoln Porphyry. The actual 
line of contact of Blue Limestone and Weber Grits, occurring generally in 
the covered gap of the depression between the ridge and the steeper mount- 
ain slope, was seldom observed. It was therefore impossible to determine 
whether the sheet of Lincoln Porphyry, which occurs above it on the higher 
part of the mountain mass, extended eastward as far as the foot-hills or not. 
Some outcrops of porphyry were observed which might have belonged to a 
continuation of this sheet, but no facts of sufficiently definite significance 
were obtained to justify its indication on the map. 

A good section of these outlying ridges is obtained in the narrow wind- 
ing gorge of lower Buckskin Creek for about a mile above Alma. The 
beds of the Weber Grits formation exposed along the walls of the valley, 
which lie within the forest-covered belt, show much more decomposition and 
disintegration than is found in the same beds above timber line. Thev con- 
sist of coarse micaceous sandstones, with a considerable development of 
argillaceous shales, also micaceous, and one or two thin beds of gray lime- 
stone. Among the shales is conspicuous a black carbonaceous bed, and the 
limestone is supposed to be that which occurs at about the middle of the 
formation and which outcrops again in the wooded hills east of the Platte 
Valley. The sandstone which immediately underlies the town of Alma 
itself, and which is made up of grains of quartz about the size of duck- 
shot, with considerable muscovite, might be mistaken at a little distance 
for a decomposed granite. It shows but few bedding planes, and, though 
in excavations for buildings it stands as a straight wall, when broken down 
it crumbles at once into coarse sand. 

Buckskin amphitheater. This immense basin at the head of Buckskin gulch 
bears the same relation to Mount Bross that the Platte amphitheater does 
to Mount Lincoln, the two separating the Lincoln massive from the main 
crest of the range, with which it is connected by the dividing ridge running 
from Mount Cameron to Democrat Mountain. 

An excellent exposure of the Archean formation is afforded in its steep 
walls, which rise 1,500 to 2,000 feet from the bottom of the basin and are 
capped on the eastern side by a thin covering of Paleozoic beds. The 
rocks are mainly gneisses and amphibolites, with local developments of 


granite, through which run irregular vein-like masses of white pegmatite. 
The latter are particularly prominent on the northeastern walls of Buck- 
skin canon, a short distance above the town of Buckskin Joe. In the bottom 
of the upper part of the basin is a small lake, above which a dike of horn- 
blende diorite forty to fifty feet wide runs across the basin in an easterly 
direction from the base of Democrat Mountain and disappears under the 
debris slopes on the other side. 

At the south base of Democrat Mountain are three small lakes or tarns, 
on a raised shoulder or knoll of granite, back of which is a small raised basin 
extending to the base of the mountain. This granite is of the fine, even- 
grained type without large porphyritic crystals, almost white in color, 
and contains both biotite and Muscovite. It is traversed by many small 
veins of pegmatite, consisting of orthoclase and quartz and often having 
a regular banded structure, like that shown in Fig. 2, Plate IV, which is from 
a sketch of a bowlder standing near the lake. 

In this raised basin many eruptive dikes, mainly of porphyrite, were 
observed, only a few of which it has been possible to delineate on the map 
These porphyrites belong to the types carrying either mica or hornblende 
and mica. They occur frequently in the form of interrupted dikes. That 
found near the uppermost of the lakes contains both hornblende and mica, 
with considerable quartz, and is remarkable for the numerous fragments of 
Archean rocks included in it. One of these fragments was several feet 
square and penetrated in all directions by veins of porphyrite, in which a 
distinctly fluidal structure of the elements of the porphyrite about it could 
be observed. 

Near the middle lake is a dike of White Porphyry, a fresh and compact 
variety of the Leadville rock; fragments of the same rock are abundant in 
the dtibris pile at the head of the gulch. 

One of the porphyrite dikes, which dips 30 to 40 north, can be traced 
to the south shoulder of Democrat Mountain, which forms the divide 
between this and the Platte amphitheater, and apparently connects with the 
long dike, which can be traced as a thin black line high up along the east- 
ernwall of the latter. A double dike of similar appearance occurs further- 
south on the same divide, near the north base of Mount Buckskin. 


On the south wall of this raised basin under Mount Buckskin the white 
granite disappears and is replaced by gneiss and hornblende schists, which 
show a remarkably contorted structure. Running nearly parallel to this 
wall, and forming its face in certain parts, is a dike, thirty to forty feet 
wide, of mica-diorite. It projects out into the valley in the direction of 
the Red amphitheater, but could not be traced on the east side. 

North Mosquito amphitheater. The Arcliean exposures at the head of the 
north branch of Mosquito gulch may properly be mentioned here. They 
consist of the same general character of rocks gneiss, schist, and granite. 
On its north wall the irregular shading of the dark mass produced by the 
white pegmatite veins is particularly prominent. The coarse-grained red 
porphyritic granites are more common lower down the canon, while towards 
the crest of the range the fine-grained, eruptive-looking granite is found, 
and apparently extends through to the west side at the head of Bird's Eye 

In the neighborhood of the little lake in this basin many dikes, often 
of the interrupted form, were observed, the more important of which have 
been indicated on the map. East of the lake, in the center of the amphi- 
theater, is a dike of Mosquito porphyry, similar to the dike No. 2 on the south 
face of Mount Lincoln, though of somewhat lighter color, owing to a differ- 
ence in the mica, and containing more ore in small specks. The oxidation 
of this ore gives a brown color to the weathered feldspars, which when fresh 
have a faint pink color. Under the microscope the groundmass is seen to be 
fully microcrystalline. The apatites are dusty. The only mica seems to 
be muscovite, which, judging from the associated yellowish grains and rarer 
needles, has come from biotite. Part of the ore seems to be magnetite, and 
part is entirely decomposed. The quartz grains contain fluid inclusions. 
This porphyry is cut in one place by a mica-porphyrite, which is a light- 
colored rock, containing numerous feldspars in a dark-gray groundmass. 
with some hexagonal plates of dark-brown biotite. Quartz, in quite large 
grains, can be distinguished by close examination. Single grains of pyrite 
are scattered through the rock. Striations are distinctly visible on many 
feldspars. Under the microscope plagioclase is seen to largely predominate 
and the biotite to be quite fresh. The groundmass, which is fully crys- 


talliiie, is composed of very uniform minute grains of quartz and feldspar, 
with mica in opaque dots, and intrudes in bays into the quartz grains, which 
are quite free from inclusions. 

To the northeast of the lake a considerable body of quartz-mica por- 
phyrite was observed; but its exact form, whether a large dike or an iso- 
lated mass, could not be determined. It closely resembles the rock already 
described from the knoll south of Democrat Mountain. It is extremely 
fine-grained, but of very dark color, owing to the large amount of biotite, 
and contains no hornblende. Many other eruptive dikes were observed on 
the face of the cliffs, which time did not admit of studying carefully. Prom- 
inent among these is a dike or sheet of dark porphyrite, cutting the ridge 
which divides the upper part of the amphitheater into halves. 


This division includes the eastern slopes up to the' crest of the range, 
from the line of lower Buckskin Valley south to that of Horseshoe or Four- 
Mile Creek. The region is crossed diagonally by the line of the London 
fault, which divides it into two parts in such a manner that there is a repe- 
tition of the same series of sedimentary beds exposed in the- canon sections 
on given transverse lines. 

Glacial erosion. Evidences of glacial erosion are abundant in the valleys 
of all the streams flowing from the crest of the range, but the data afforded 
by Buckskin and Mosquito gulches is so definite as to seem worthy of 
special mention. As the map shows, the two valleys are nearly parallel 
and similar in general form, in that their main course in the Archean rocks 
is southeast, the glaciers which originally filled them having been fed by a 
very broad ne've'-field, filling two or more less distinct basins at their head, 
and that in their lower course, where they reach the upturned edges of the 
Paleozoic strata at the line of their steepening dip, they bend sharply to 
the east and cross these strata approximately at right angles to their strike. 
Just above the bend a raised bench or shoulder is found on the south side 
of either cafion, several hundred feet above its present bottom, which is evi- 
dently a portion of a former valley bottom, and marks approximately the 
level to which the valley was cut out by the glacier which once filled it. 


In Buckskin Canon this bench, which has been cut across by several 
minor ravines, is not sufficiently regular to be defined by the contours of 
the map, although it is readily apparent to the eye. That in Mosquito 
gulch, however, which forms a practically continuous terrace nearly a mile 
and a half in length on the north face of Pennsylvania Hill, is shown by the 
topography of the map, and is about seven hundred feet above the bed of 
the present stream. It has about the same slope in an easterly direction 
as the present valley, and this slope carried upward corresponds with the 
present bottom of the South Mosquito amphitheater above the London fault, 
which is formed by gently-dipping quartzites and schists of the Weber 
Grits formation whose angle over a considerable area is about the same as 
that of the bottom of the basin. On the rock surfaces of the flat portion of 
this basin glacial grooves and strife are still distinctly to be seen, showing 
that but little erosion has taken place since the Glacial epoch. On the other 
hand, in the neighborhood of the fault and in the Archean rocks below it, 
the present stream-bed deepens very rapidly and the valley becomes a nar- 
row, winding, V-shaped gorge. In the north Mosquito amphitheater, which 
is entirely in Archean rocks, the upper part of the basin (which, owing to 
its great elevation and the consequent low temperature that prevails in it, 
suffers but little abrasion by running water) remains at essentially the same 
level as the South Mosquito amphitheater, but the V-shaped cutting by 
present streams extends back much farther than in the latter. The conclu- 
sion to be drawn from these facts is that the eroding force of glacier ice is 
a power so great as to be comparatively independent of the materials on 
which it acts, while that of running water varies very greatly with the dif- 
ferent forms and characters of these materials. Thus the original glacial 
cutting of lower Mosquito gulch formed a comparatively straight and regular 
valley, but the 1 present stream-bed near the mouth of the canon makes a 
bend to the south, around a boss of more resisting granite on the north side 
of the valley, and then is deflected to the north by the upturned edges of 
the Paleozoic strata which cross its course diagonally. 

The Mosquito glacier, as might be expected from its course, left its 
moraine material mainly on the south side of the valley, where it forms 
several wooded ridges opposite Park City. It was of greater extent than 


the Buckskin glacier, and probably once reached down to the Platte, the 
actual bottom of the present canon being from one hundred to two hundred 
feet lower than corresponding portions of Buckskin gulch. Both Mosquito 
and Buckskin gulches open out into alluvial bottoms below the catton 
mouth, but the stream in the latter soon runs into a narrow, winding gor-e, 
which extends for a mile above Alma. The connection of the Buckskin 
glacier with the Platte glacier, if it ever existed, must, therefore, have been 
above the low ridge through which this gorge is cut. 

Buckskin section. The most complete and instructive sections of the lower 
Paleozoic beds and their included sheets of eruptive rock are obtained on 
the walls of the cafions near their mouths, just before the beds dip down 
more steeply to the east and disappear beneath the softer slopes of the 
lower rounded hills or are covered by the alluvial deposits of the streams. 
That on the south .side of Buckskin gulch, just about the deserted town of 
Buckskin Joe, is represented by the diagrammatic sketch given in Plate XIII. 
The total height of the cliff above the valley bottom is here about one thou- 
sand feet. 

The Archean exposures (a), occupying the lower portion, are largely 
concealed by huge talus slopes of debris, which in some places extend up 
so high as to cover the base of the Cambrian, while the Blue Limestone at 
the top of the cliff is covered by soil. The portion represented in the 
sketch shows, therefore, only the Cambrian and Silurian beds and the 
manner of distribution of the intrusive sheets of porphyry and porphyrite. 
These are here very irregular as compared with sections elsewhere, which 
is doubtless due to the fact that they are near the northern limit of the bodies, 
and hence that the intrusive power which forced them between the beds was 
already diminishing in energy. The upper bed is about fifteen to twenty ' 
feet in thickness and consists of dark-green hornblende-porphyrite, of the 
typical variety already described from Mosquito gulch. As shown in the 
plate, it varies in thickness and often wedges out, its continuation occurring 
farther on at a slightly higher or lower horizon. At its contact with the 
bounding sedimentary rocks it becomes more compact, but the sedimentary 
beds show no caustic phenomena, though they are sometimes slightly con- 
torted. About thirty feet below this is a second intrusive sheet, also 

;<;:* -.-.-? .;;, ; .<>.. 

.V--V- ,'.->,, -*' .-J-J, 

;: -., -;.. 




' v 'ffW - " 


very variable in thickness, of gray quartz-porphyry, like the Lincoln, but 
without its large feldspars and with its basic silicates generally much altered. 
Between this and the Archean are forty to fifty feet of white saccharoidal 
quartzite, with a thin bed of fine-grained conglomerate at the base, wherever 
the base can be distinguished. The Archean here consists of a dark mica- 
gneiss, approaching a mica-schist in structure. 

The dark, more or less perpendicular lines on the sketch represent 
shallow ravines on the face of the cliff, which are generally fracture planes 
across the beds, accompanied by a certain amount of dislocation. The 
principal ravine is that to which the double line over the de'bris pile (which 
represents a raised tramway for carrying down ore) leads, and in which are 
the now deserted workings of the Northern Light mine. This fault had a 
movement of about fifteen or twenty feet, and the ore seems to have been 
found in the crevice of the fault. These small faults were probably pro- 
duced by the general dynamic movement in which the rocks were folded, 
and it will be noticed in the sketch that the intrusive sheets are faulted in 
the same degree as the inclosing sedimentary beds. About half a mile west 
of the point represented on the sketch is a prominent fault on the cliff, with 
an upthrow to the west of about one hundred feet. The direction of this 
fault, as of the minor fracture planes in the sketch, is between north and 
northeast, which corresponds with those observed near the Criterion mine, 
on the opposite wall of the gulch. 

East of the Northern Light mine the beds slope rapidly down in a 
graceful curve to the bed of the gulch, in which only the outcrops of the 
harder and more silicious beds project above the gravel. The former min- 
ing town of Buckskin Joe, the oldest settlement in this region (now, like its 
companions, Quartzville and Montgomery, consisting mainly of deserted 
cabins and mill foundations), is situated on the outcrops of the base of the 
White Limestone. On the south side of the creek, a little above the town, 
is the once famous Phillips mine, an open trench, some twenty feet wide and 
in places as many deep, cut in an immense concentration of iron pyrites along 
a bedding plane of the Cambrian quartzite. In one place a decomposed 
quartz-porphyry is found on the hanging wall, which apparently cuts across 
the formation, as it is also found in the creek bed near the bridge at a some- 



what higher horizon than the ore body. This porphyry resembles the rock 
of the lower intrusive sheet shown in the sketch, and may form part of it, 
though it was not possible to trace the connection between the two. 

Loveiand Hill Loveland Hill affords an excellent illustration of the often- 
observed fact that the deeper transverse valleys often follow the line of a 
minor or lateral anticlinal fold, while the intermediate hills or more ele- 
vated region, which has been relatively less eroded, is the locus of a minor 
synclinal fold. 

On the broad, flat back of this hill or spur, whose slope corresponds 
very nearly with the easterly dip of the sedimentary beds, is a shallow 
ravine draining into Mosquito gulch, towards which there is a very per- 
ceptible dip of the beds from either side; in other words, the strata dip 
eastward, and at the same time dip north and so'ith towards the bottom of 
this valley. The larger part of the surface of the hill is covered by beds 
of Blue Limestone The White Limestone comes to the surface at its upper 
end, and on the sharp ridge which separates the north Mosquito amphi- 
theater from Buckskin gulch are the remains of the lowest quartzite beds 
of the Cambrian. The Blue Limestone has been extensively prospected for 
ore, and a number of irregular deposits have been discovered, generally 
occupying gash veins, or cross joints and fault planes in the limestone. 
Numerous irregular bodies of porphyry are also found. Time did not 
admit, however, of a complete study of these beds nor of the ore deposits. 
The principal facts ascertained will be found in the description of mines in 
Part II, Chapter V. 

The synclinal ravine already mentioned divides the hill somewhat un- 
equally into a northern and a southern portion. The former forms a con- 
tinuous ridge, which extends down to the junction of Mosquito Creek with 
the Platte River below Alma. East of the mouths of the canons this ridge 
is comparatively low and covered with forests and soil. It is made up of 
beds of the Weber Grits formation, in which there is evidence of a sec- 
ondary roll, as shown in Section C, Atlas Sheet VIII. Along the steeper 
slopes of the spur between Buckskin Joe and Park City are outcrops of a 
body of quartz-porphyry of the Lincoln type, which apparently forms a 
sheet above the Blue Limestone. These outcrops are not very continuous, 


but it seems probable that they are the remains of a sheet that once cov- 
ered Loveland Hill in an analogous manner to the porphyry sheets on 
Mounts Bross and Lincoln. 

By the erosion of the synclinal ravine above Park City the White Lime- 
stone is exposed in its bed with some irregular bodies of porphyry, and the 
southern half of Loveland Hill, south of this ravine, ends to the eastward in 
a cliff, at the base of which are exposed quartzites, apparently of the Cam- 
brian formation, in which several ore bodies have been found. From the 
base of this cliff the formations sweep in a curve across Mosquito gulch 
and up the north face of Pennsylvania Hill. The Cambrian and Silurian 
outcrops can be traced in the bed of the gulch, dipping eastward at angles 
of 20 to 25, but the Blue Limestone outcrops are concealed by gravel 
and alluvial deposits in the widening valley below. 

North Mosquito section. The cliffs on the south face of Loveland Hill afford 
a section of the lower Paleozoic, with their included intrusive sheets, simi- 
lar to but even more perfect than that on its northern face toward Buckskin 
gulch. Thin sheets of interbedded porphyry and porphyrite can be traced 
along them for nearly two miles in practical continuity. The fault which 
was observed on either side of Buckskin gulch is not found on this cliff 
wall, but near the mouth of the canon is a more remarkable fault, whose 
direction is at right angles to the one above mentioned. Seen from the other 
side of the canon, the strata seem to slope rapidly eastward until they abut 
against the western side of a little knoll of granite, which projects out into the 
valley at this point and deflects the stream to the southward. When one 
actually climbs the cliff, however, it is found that there is a reduplication of 
the lower part of the beds ; that a faulting has sheared or split off a portion 
of the strata on a southeast line, nearly parallel with the face of the cliff; 
and that tlie piece thus separated has apparently fallen down at its eastern 
end to the base of the cliffs, while at its western end it still maintains its 
connection with the regular line of outcrops. In Plate XIV is given a dia- 
grammatic sketch of a portion of this cliff toward the eastern end, where the 
steeper dips come in. In the foreground may be seen the faulted-down beds 
referred to above, which form a low ridge or shoulder, standing out a little 
distance from the face of the cliff. Above and back of this ridge the main 
cliff rises nearly perpendicularly, showing the regular series of Cambrian 


and Silurian beds above the Archean, the softer covered slopes on the top 
of the ridge being underlaid by the Blue Limestone. The section from the 
commencement of actual cliff slope downwards is as follows : 


Siluii.m / W |itc Liuiest.oiie, not measured. 

\ Porphyrite, 25 feet. 

Quartzite aud shales 50 

Porphyry, 20 feet. 

Quartzite 50 

Quartz P r P b ro'i 10 feet 

Cambrian <( 

Quartzite 25 

I Altered quartz-porphyry, 15 feet. 

j Quartzite with flue conglomerate at base ... 15 


Archean Gueiss 

The upper intrusive bed is the normal hornblende-porphyrite, found 
also on the opposite side of Mosquito gulch, and already described in the 
Buckskin section. This bed, as will be observed in the sketch, is at the 
base of the White Limestone on the right, and above this horizon in the 
White Limestone on the left. It does not, however, break the continuity 
of the sedimentary beds in the plane of this section as it passes from one 
horizon to another, but it wedges out at one horizon and comes in again 
a little further on, also in a wedge-shaped body, at a slightly higher horizon- 
The rock of the second intrusive bed has also the external characters of 
a porphyrite, and has been indicated as such in the sketch ; but micro- 
scopical examination shows it to belong to the Green Porphyry type. The 
third bed is a true quartz-porphyry, resembling the Lincoln Porphyry, but 
without its large feldspars, and corresponds to the sheet in the Cambrian 
on the Buckskin section. The lowest sheet is also a quartz-porphyry, but 
so much altered that not much can be said as to its probable type. 

On the faulted-down ridge at the foot of the cliff the Green Por- 
phyry forms the top rock, and the main bed of quartz-porphyry below 
can be readily traced ; but the lower one is less distinct. 

The chimney -like ravines which furrow the face of the cliff probably 
follow, as on the Buckskin section, fracture or fault planes. In the one 
which was examined, the left hand of the three from which the debris piles 
descend in the sketch, there is a discrepancy of about six feet in the beds 
on either side. 



It is to be remarked that, as the scale of the map and sections was too 
small to show all the intrusive sheets mentioned above, they have there 
been generalized into two bodies, one of porphyry and one of porphyrite. 

south Mosquito section. On the south wall of Mosquito gulch, opposite the 
cliff described above, a similar and equally instructive cliff section is found, 
in which however there is no great fault ; only the slight dislocations marked 
by shallow ravines, common to all the cliff sections. The beds on this cliff 
were examined in some detail and careful measurements were taken, as it 
was considered to be a type section. The series from the top of the cliff 
downwards is: 


r Coarse granular gray limestone i with black chert 

I Blue lighter-colored limestone. 1 seams 60 

Lower Carboniferous . 1 Light-bluish limestone, weathering yellow 30 

I Blue limestone, generally thin-bedded 40 


White Parting Quartzite, heavy-bedded, coarse at 

top 40 

Porphyrite,2 feet, thickeningto 20 feet farther west. 
Silurian </ Limestones, light blue at top, gray semi-crystalline 

below; more silicious and shaly towards base 100 
Very thin blue clay-shales, with shaly quartzite. . . 10 
White quartzite, more or less calcareous 30 


Shales, argillaceous and silicious, containing red- 
cast beds 12 

Thin-bedded quartzite, with shale beds few inches 
thick : 14 

White saccharoidal quartzite 18 

Porphyrite, G feet (thickens to the eastward). 

White saccharoidal quartzite, in beds from 4 inches 

to 4 feet 30 

Cambrian ^ Quartz porphyry, 30 feet. 

White saccharoidal quarrzite, massive above, thin- 
bedded toward base; often discolored red and 
brown on surface 35 

Quartz-porphyry, altered, 7 feet. 

Quartzite, iron-stained 1 

Quartz-porphyry, altered, 8 feet. 

White quartzite, conglomerate at base 40 


. , i Gneiss, rich in mica 

' ( Granite, with red tabular feldspars 


In the above section the top of the Blue Limestone was possibly not 
reached, as it forms the surface of the hill, and may have been partially 
removed by erosion. The thickness given of 130 feet is much less than is 
found in the vicinity of Leadville. It is readily seen from the varying 
character of the beds at the base of the Silurian and at the top of the Cam- 
brian that, in the absence of paleoutological evidence, it is difficult to draw 
a definite line between the formations. These beds were deposited at a 
time when the.general character of the sediments was changing from sili- 
cious to calcareous, and the rapidity with which the change progressed natu- 
rally varied much within comparatively short distances. The Red-cast bed, 
of which a specimen from this section is figured in Plate V, is the only one 
whose character is found to be persistent over the whole area, and this has, 
therefore, been adopted provisorily as the top of the Cambrian. The aver- 
age strike of the beds is north and south, and the dip varies from a very 
low angle to 25 east. 

The rock of each of the porphyrite beds is of the typical hornblende 
variety figured on Plate VII, Fig. 2. As in other sections, while the 
porphyrite is continuous on a large scale throughout certain horizons, in 
detail it is found to be very variable in form, now ending on one bedding 
plane in a tongue, around which broken masses of the sedimentary beds are 
distributed like material pushed before the end of a lava flow, and then 
continued a few feet farther on another bedding plane. Again it appears 
in small transverse dikes, probably offshoots from the interbedded sheets. 
Of these the most prominent is at the horizon of the Red-cast beds, standing 
vertically, with an east and west strike, and 10 feet thick. There are two 
main sheets of porphyrite. The upper one is only two feet thick in the line 
of section, and occurs between the Parting Quartzite and White Limestone. 
As it rises with the slope of the beds to the westward it gradually thickens, 
becoming 17 to 20 feet thick at the point where it reaches the top of the 
cliff, and here occurring between the Blue Limestone and Parting Quartzite. 
The second sheet occurs in the upper part of the Cambrian, being only six 
feet on the line of section, but thickening to the eastward. 

The rock of the porphyry sheets is so much decomposed that it cannot 
be definitely decided whether it is more closely allied to the Lincoln or to 


the Sacramento type, occurring as it does in geographically debatable 
ground, or about at the limits of the extent of either variety. They occur 
in the lower part of the Cambrian, the .upper sheet being 30 feet thick on the 
line of the section, above which is a long dike about three feet thick, proba- 
bly an offshoot from it. The lower sheet, which on this line has a thin bed 
of quartzite included in its mass, is 15 feet thick, and is found a little farther 
west without any included quartzite. This lower porphyry sheet extends 
westward along the north face of Pennsylvania Hill as far as the London 

Pennsylvania Hiii. This name has been given to the broad, flat-backed 
spur included between Mosquito and Big Sacramento gulches. Like its 
neighbor, Loveland Hill, it is the locus of a slight synclinal fold, which 
forms a shallow ravine on its back drained by Pennsylvania Creek Except- 
ing along the cliff walls of the adjoining canons, it affords but few good rock 
exposures, since its surface and that of the spurs which run down from it 
to the valley of the Platte are densely covered with forest growth and soil. 
The varying direction of dips observed in the sandstones of the Weber 
Grits which form the lower spurs gives evidence of one or more secondary 
rolls or folds in the outlying strata, as indicated in somewhat generalized 
form on Sections D and E, Atlas Sheets VIII and IX. The most definite 
evidence is found on the hill south of Park City, known to the miners as 
Baldhead. The northeast slopes of the hill and many of the lower hills 
extending eastward from it are made up of moraine material from the ancient 
Mosquito glacier. The various porphyry bodies found in this wooded re- 
gion, of which only the more prominent are indicated on the map, are gen- 
erally very much decomposed. When their character could be still recog- 
nized they were found to belong to the Sacramento type. They have gen- 
erally a greenish color, due to the peculiar alteration of the basic constitu- 
ents of the rock. Above timber-line the slope of the hill corresponds so closely 
with that of the stratification planes that good outcrops are only to be found, 
as a rule, on the cliff faces to the north, west, and south. The shallow ravine 
on its back divides it into two portions, on the northern of which the beds 
have the prevailing strike already observed, viz, about north and south. On 
the southern portion, on the other hand, the strike is about 20 west of north, 


and to this change of strike the synclinal structure observed may be in part 

Along the northern wall, west of the Mosquito section just described, 
the Cambrian and Silurian strata form a thin capping to the Archean cliffs. 
At the western point of the hill decomposed porphyry is still traceable in 
the Cambrian, but at the very highest point, near the line of the London 
fault, only White Limestone is found at the surface. This can be seen to 
bend over in an anticlinal fold before it is cut off by the fault, and a promi- 
nent quartzite crag, which will be described later, is assumed to be a portion 
of the Parting Quartzite which has escaped erosion, standing in a vertical 
position on the west side of this anticline and adjoining the fault plane. 

On the south face of the cliff overlooking Big Sacramento gulch an 
eminence of the ridge just east of the fault line is capped by a body of 
Sacramento Porphyry about one hundred feet in thickness. Over this, on 
the eastern flank of the ridge, whose slope is but little steeper than that of 
the strata, are further beds of white quartzite, succeeded lower down by 
the overlying White Limestone. This white quartzite therefore represents 
the Cambrian formation, and the Sacramento Porphyry an interbedded sheet, 
here locally developed in unusual thickness. At the foot of the steeper 
eastern slope of the ridge is found the Blue Limestone, over which is a bed 
of decomposed Sacramento Porphyry, almost identical with that which is 
characteristically developed in the Sacramento mine on the same horizon. 
Zones of decomposition are characteristically marked on this rock by con- 
centric lines, stained red by oxide of iron, the very kernel of the larger 
blocks sometimes, though rarely, showing the original bluish color of the 
unaltered porphyry. 

London fault. The region thus far described has been one comparatively 
free from faults, the movements of displacement being, as it were, within the 
beds, and generally not more than one hundred feet in amount; move- 
ments which have exerted no perceptible influence on the character of the 
topography and have made comparatively little change in the geological 

South of Mosquito gulch the eastern slopes of the range are divided 
by one great fault line running diagonally across them, and finally dying out 


at the southeastern corner of the map. This is the London fault, so called 
from the hill dividing the two heads of the Mosquito gulch, through which 
it passes. Its effects can be readily traced by the traveler who approaches 
Leadville over the Mosquito pass. The Mosquito pass road, following up 
the valley bottom of the north fork of Mosquito gulch, winds up the steep west 
wall of the gulch and, passing through the narrow notch between London 
Hill and the main crest of the range, ascends gradually in a southwest direc- 
tion to the Mosquito pass. Up to the point where it reaches the northwest 
wall of London Hill the rocks around are of gneiss and granite ; from there 
to the summit of the pass is a confused mass of huge loose blocks of coarse 
quartzitic sandstone and fine-grained porphyry, in which it requires a trained 
eye to distinguish any definite structure lines, although the change in the 
character of the rocks is evident to all. Looking south from the road across 
the broad basin of the South Mosquito amphitheater, the eye is at once 
attracted by the peculiar appearance of the ridge which forms its south wall. 
This is the summit of Pennsylvania Hill. As seen from this distance, paral- 
lel with the regular and comparatively gentle slope of its surface eastward 
there can be distinguished along its upper wall a few horizontal lines mark- 
ing the bedding planes of the Paleozoic strata, below which the steep face 
of the hill shows no definite structure lines on its rocky surface, save those 
which mark the talus slopes of broken rock accumulated towards its base. 
The smooth, regular slope is broken at its crest by a dark knob, around 
which the rocks are greatly discolored, and the debris from which presents 
brilliant hues of yellow and red. West of the knob the outline of the hill 
presents terrace-like escarpments, descending nearly to the level of the 
amphitheater. The face of this portion of the hill shows regular stratifica- 
tion lines, dipping eastward at an angle of 20, which can be seen with 
great distinctness to its very base, where they are concealed by the talus 
slopes. All end abruptly to the east before reaching the discolored knob. 
This break in the continuity of the stratification lines, or of the beds which 
they outline, is evident at the first glance, as marking the line of a great 
fault plane. The evidence of the existence of this fault can be seen with 
equal distinctness on the walls of the canon gulches to the south of Mos- 
quito gulch, although in either case the conditions vary, both in the hori- 


zon of the juxtaposed beds on either side of the fault and in other struct- 
ural outlines. Although its existence is so evident, yet its actual position 
cannot be determined with absolute accuracy, the possible error of location 
varying under different conditions from ten to one hundred or even two 
hundred feet. The reason for this uncertainty is found in the fact that the 
surface rocks in the immediate neighborhood of the fault are generally so 
much altered and decomposed that their structure planes cannot be traced, 
and that the fault plane has not been cut by any underground explora- 
tions. The direction of the fault, as determined from points where it crosses 
ridges or gulches, varies from N. 15 W. to N. 45 W., its average direction 
being N. 30 W. or NW. magnetic. The great S-shaped anticlinal fold 
which is everywhere found in close proximity to the fault lias the same 
general direction ; nevertheless the two directions do not seem to be coinci- 
dent for any long distance, but diverge a little from each other, so that the 
fault cuts the fold, now in one part, now in another, but generally west of 
the anticlinal axis. Thus from Pennsylvania Hill to Sheep Mountain it 
corresponds closely with the axis of the syncline to the west of the great 
anticline ; south of Sheep Mountain it gradually approaches the anticline, 
until at the extremity of the ridge both fault and fold die out. North of 
Pennsylvania Hill the line of the fault has a more easterly direction than 
that of the fold, and on London Hill it cuts the fold east of the synclinal 
axis, and a little beyond it may very nearly coincide with the anticlinal axis ; 
but, as the sedimentary beds have been entirely eroded away from above the 
Archean, it is no longer possible to determine the position of this axis. The 
amount of displacement occasioned by this fault can only be determined 
approximately, since the fact of its near coincidence with the anticlinal 
fold introduces an unknown factor, viz, the amount of apparent displace- 
ment that may be due to actual plication. The reason of this can best be 
understood by reference to Sections C, D, E, F, G, and H, on Atlas Sheets 
VIII and IX, which are drawn to scale and have been constructed with 
great care from observed outcrops, dips, and thicknesses of formations. 
The movement of displacement, as shown in these sections, which is prob- 
ably a minimum, averages a little over two thousand feet. 


The description of the geological character of the region west of the 
fault will now be resumed in topographical order as it has been carried on 
hitherto, taking up alternately the canon sections and intermediate ridges in 
regular succession as one goes south. 

Main crest from Mosquito Peak to Mount Evans. Oil the main Crest of the rangfe, 

o ' 

at the head of the south half of the North Mosquito Amphitheater, the fault 
line is well marked by a sudden change from limestone to a coarse red 
granite, in the saddle or notch between Mosquito Peak and the peak next 
north of it. The upper tunnel of the Little Corinne mine, on the north face 
of Mosquito Peak, is run near the top of the Blue Limestone; above this are 
1'2 feet of White Porphyry, while the lower tunnel of the same mine is in 
White Limestone. The shales and quartzites of the Weber Grits formation 
form the summit of the peak, included in which is a thin bed of Sacra- 
mento Porphyry. 

From Mosquito Peak southward to Mount Evans the main crest of 
the range is a nearly straight ridge, steeply escarped on the west. At 
the base of this escarpment runs the Mosquito fault, by whose displace- 
ment the Archean schists have been thrust up into juxtaposition with the 
beds of Weber Grits formation on the west. The beds of the lower Paleo- 
zoic series can be traced along the summit of this wall, descending gradually 
to the southward, until under Mount Evans, in the Evans amphitheater, they 
are found at the base of the slope. In this extent there is a slight break 
in their continuity, occasioned by a transverse fault in a little ravine just 
south of the zigzags of the Mosquito grade. The thin sheet of White Por- 
phyry lying above the Blue Limestone, which is observed at Mosquito 
Peak, disappears before reaching Mosquito pass; but the sheet of Sacra- 
mento Porphyry 10 feet thick, which occurs in the lower portion of the 
Weber Grits, apparently at the summit of the shale division of this formation, 
gradually thickens to the southward, and on the eastern wall of the Evans 
amphitheater it suddenly widens out into a body five hundred to seven hun- 
dred feet in thickness. Owing to the sharp contrast of the angular and 
almost Gothic forms, into which this mass weathers, with the horizontal 
lines of the bounding sedimentary beds, its outlines can be readily distin- 
guished even from so great a distance as Leadville itself, and would be seen 


in the heliotype view on page 6 had the photographic picture been equally 
distinct with that which is formed on the eye of the observer. At the point 
where the Mosquito grade descends this steep western wall the Lower Quartz- 
ite comes in contact with Weber Grits on the west of the fault, but to the 
north and south of this point Archean exposures intervene between the base 
of the Paleozoic and the line of the Mosquito fault 

On the crest of the ridge are two irregular-shaped bodies of Sacramento 
Porphyry at a higher horizon than the sheet already mentioned, which are 
supposed to be the relics of a second intrusive sheet. The first of these 
forms the summit of the peak next south of the Mosquito Peak, and can be 
traced down its eastern slope across the Mosquito grade. The second forms 
the crest of the ridge for some little distance south of Mosquito pass. In 
the saddle west of London Hill the road crosses another exposure of por- 
phyry, which is supposed to be the outcrop of the lower sheet of Sacra- 
mento Porphyry exposed along the western face of the crest; while in the 
sharp, prow-like point of London Hill is another interbedded sheet of Sacra- 
mento Porphyry, which, as indicated in Section C, is presumed to be a con- 
tinuation of the upper body, which is found on the crest of the range. 

South Mosquito amphitheater. The bed of this basin is formed by coarse 
sandstones and grits of the Weber formation, dipping 20 to the east with 
its slope. On the exposed faces of these beds glacial grooves and striae 
are often very distinct. In the sandstones are various beds of porphyry, 
and among the debris piles of huge rock fragments split off by ice and 
frost, which form the steep slopes of the eastern and southern walls, por- 
phyry forms an important element. Time did not admit of tracing out 
the outlines and relations of all these porphyry bodies, and the structure 
given on the map and sections, which assumes that the lower sheet which out- 
crops along the west side of the crest of the range once extended over the 
whole basin, may be only partially correct. 

Sacramento amphitheater. Big Sacramento gulch, like those to the north of 
it, was once occupied by a glacier, and the amphitheater at its head, like 
that of the South Mosquito, has been probably but little deepened since 
Glacial time. The deeper cutting of the present stream extends some little 
distance above the fault; below the fault line the bottom opens out into 


springy meadows and then closes together, as it bends to the southward, 
between gravelly ridges which are evidently the remains of former 
moraines and which extend below the junction with Little Sacramento. 
Owing to the dense growth of forest on these ridges, however, the actual 
lower limits of the glacier are not easily determined. About a mile above 
the line of the fault the narrow bottom of the present stream ends in shelf- 
like terraces of white sandstone, above which the valley opens out into the 
broad basin of the Sacramento amphitheater. On the face of this terraced 
wall, and about opposite the western point of Pennsylvania Hill, are two 
dolomitic limestone strata: the lower one, a dark-gray semicrystalline rock, 
with clayey seams, is about ten feet in thickness; the second, sixty feet 
above this, is only six to eight feet in thickness, of similar color and also 
associated with clay shales, the intervening beds being of coarse Weber 
sandstones. Among the fossils found here were identified 
Prodnctus costatus and Athyris subtilita. 

Ascending the stream farther, successive beds of white sandstone are 
crossed until the great body of Sacramento Porphyry is reached, which in 
a probable thickness of twelve hundred feet forms either wall of the amphi- 
theater. The upper extremity of the amphitheater was not explored, but 
from information and specimens furnished by Mr. J. T. Long sufficient 
evidence was obtained to justify the indication of an outcrop of Blue Lime- 
stone below the Sacramento Porphyry at its deepest part. The fossils 
obtained by him from here, besides the uncharacteristic Athyris subtilita, 
included the new Spirifera, like Spirifera Kentuckensis, which has not yet 
been found at a higher horizon than the Blue Limestone. Among minerals 
small yellow crystals of pyromorphite were found with the specimens of 
ore obtained from this horizon. 

London Hill. The line of the London fault crosses London Hill diago- 
nally about seven hundred feet west of the summit, in such a manner that 
the greater part of the steep northern slopes is occupied by Archean rocks, 
with only the extreme eastern end made up of easterly dipping quartzites of 
the Weber Grits formation, whereas on the south side the latter extend over 
two-thirds of the lower slopes. From the saddle north of Mosquito Peak 
the London fault runs southeast to a point in the raised basin north of the 


London mine, then bends more to the southward across London Hill. Under 
Mosquito Peak the beds lie in a shallow synclinal, with the Blue Limestone 
rising up gently to the eastward against the line of the fault. On the south- 
east slope of this peak the limestone forms a cliff wall, rising abruptly above 
the granite on the other side of the fault, thus affording another illustration 
of the fact that flat beds resist erosion more from the fact of their horizon- 
tality than from any greater resisting power of the materials which com- 
pose them. Half way between Mosquito Peak and London Hill, near the 
New York mine, a thin bed of White Porphyry is found at the base of the 
cliff under the limestone; the outcrops of the formations cannot be traced 
continuously to London Hill, as its lower- slopes are covered by a great 
thickness of debris. 

The London mine at the time of visit was opened by two tunnels, one 
above the other, a short distance west of the line of the fault. The lower 
tunnel, at the base of the hill, after passing through a great thickness of 
debris, consisting of large rock fragments frozen into so solid a mass as to 
require blasting, follows the stratification planes of nearly vertical beds of 
light-colored limestone, whose strike is a little more to the west of north 
than the direction of the fault plane. The dip of the strata is a little wot 
of the vertical. Between the beds of limestone is a compact White Por- 
phyry, which can in the mine hardly be distinguished from the limestone, 
especially as it effervesces with acid ; it contains, however, occasional dark 
flakes of mica, and chemical tests placed its character beyond a doubt, 
though it contains a percentage of soluble matter, mainly carbonate of lime 
with a little magnesia (10 per cent, in the specimen tested), which is too 
high to have come from the decomposition of feldspar alone, and must, 
therefore, be supposed to be an infiltration from the inclosing limestone.*. 
The limestones adjoining the porphyry to the east are very light colored and 
contain over 10 per cent, of silica, which is about the normal percentage of 
the upper part of the White Limestone. As the ore deposits follow the 
stratification planes, not much exploration has been done across the strata, 
and owing to the metamorphosed condition of the rocks exact determina- 
tions of horizon were not practicable. It may be assumed, however, that 
the ore deposits of the London mine occur in the upper part, if not at the 


very top, of the White Limestone. On the hill above, it can be seen that the 
fault line crosses the ends of the upturned strata at a very acute angle. ' 

The point where the easterly-dipping Weber Grits beds change their 
inclination to a sharp western dip, as they must to allow of the coming up of 
the underlying Blue and White Limestones, as shown in Section C, is not 
very sharply defined. Some beds of Blue Limestone can be distinguished 
between them and the fault line, but, while there was not time for exact 
measurements, and these could hardly have been made without a map, which 
was entirely wanting at the time of field work, it seems most probable that 
these upturned beds have been actually compressed against the fault plane 
to a smaller thickness than they have in a more horizontal position. 

The southwest slopes of London Hill contained no mine openings, and 
were too much covered by soil and debris to show clearly defined structure 
lines, though the sandstone beds of the Weber Grits formation were seen 
to change their dip from 20 to 50. At the point where the old wagon road 
descends into the deeper valley of the south fork of the Mosquito gulch the 
actual fault line can be distinguished, a tunnel having been run in the decom- 
posed and highly metamorphosed slates and quartzites, which here directly 
adjoin the granite beyond the fault. This point of contact bears only 10 
W. of N. from the dark crag on Pennsylvania Hill, which is nearly on the 
line of the fault. It is evident, therefore, that there is a sharp bend in the 
direction of the fault at this point, even more marked, perhaps, than that 
which is indicated on the map, though, as the position of the tunnel has not 
been determined instrumentally, nor the old road located on the map, it is 
not possible to fix absolutely the position of this bend. Here for some 
distance to the west of the fault line the strata stand not only vertically, 
but have an- inclination of 50 to the west; the strike,however, is Approxi- 
mately the same as elsewhere, viz, about N. 20 W. Thicknesses of about 
two hundred feet of vertical strata are exposed, so much altered that their 
lithological character can with difficulty be distinguished. They include 
shales and some silicious beds, with one bed of limestone. A short distance 
to the west of the fault the characteristic sandstones of the Weber Grits 
are met, with the regular dip of 20 to the northeast. It seems evident 
that the structure here is the same as that just described at the London 


mine, viz, that these much metamorphosed and vertical beds are the lower 
Paleozoic strata coming up from under a sharp syncline, compressed and 
altered beyond recognition by the dynamic movement at the time of and 
subsequently to the faulting. This would seem at first glance to be an 
explanation inconsistent with that which is offered for the conditions which 
obtain on Pennsylvania Hill, on the opposite side of the gulch ; but the fact 
that the fault line comes in the one case east of the synclinal axis and in 
the other nearly coincides with it, and the supposition that compression sub- 
sequent to the faulting has not only produced sufficient heat to alter the 
original character of the beds, but has steepened the dips of the already 
inclined beds and actually made them thinner, sufficiently explain the appar- 
ent incongruity. 

Pennsylvania Hiii west of London fault. The western end of Pennsylvania Hill, 
through which the London fault runs, is deserving of detailed description. 
Its structure is shown in section D, with the ideal position of the beds 
in depth. The observed facts are these : Ascending the wedge-shaped 
western point of the ridge from the saddle which divides South Mosquito 
from Sacramento amphitheater, one crosses a regular series of sedimentary 
beds, dipping 20 to the eastward, with two interbedded sheets of porphyry 
apparently conformable with the sedimentary beds. The ridge has almost 
perpendicular walls both to the north and south, on which the structure 
lines can be distinctly seen. The horizon of the beds which cap the divid- 
ing saddle at the base of the ridge is estimated to be 150 or 200 feet higher 
than the limestone beds which occur about the middle of the Weber Grits 
formation. About half way up the steep western slope, which is mainly 
composed of coarse sandstone with some few intercalated beds of shale, is a 
body of interbedded Sacramento Porphyry, of a thickness of 15 to 20 feet. 
Near the top of this steeper slope is a bed of black sandstone, composed of 
white quartz sand and fine grains of carbonaceous material in the nature of 
anthracite or graphite, which is very characteristic of this formation. The 
very summit of the steeper ridge is formed by a second body of porphyry, a 
fine-grained gray rock with conchoidal fracture, resembling the Silverheels 
Porphyry, whose thickness is 25 to 30 feet. Above the steeper slope 
of the ridge the surface is nearly flat and widens out so that the succeeding 


beds can only be observed along the cliff faces. Above the porphyry is a 
body of purple silicious shales, succeeded by white sandstone, with an occa- 
sional band of black sandstone similar to that already described. Prom- 
inent among these sandstones is a very coarse conglomerate, with large 
pebbles of quartz and fragments of Archean schists and granite. As one 
proceeds east the dip of the beds steepens slightly, perhaps to about 25, 
till, on approaching within two hundred yards of the fault, it changes 
apparently with great suddenness to a practically vertical angle. At the 
same time the beds are found to be greatly decomposed and stained a red- 
dish-yellow color. These beds being much more readily disintegrated, the 
structure lines, when seen close to, become indistinct, being masked by 
debris. They consist, as well as can be determined, of shales and sand- 
stones, with one belt of blue limestone, immediately adjoining the dark 
knob on the west, which has a thickness of about eight feet, adjoining which 
is a bed of White Porphyry. The dark knob, which forms so prominent a 
feature on the north wall of the hill, is white quartzite, 50 feet or more 
in thickness, which on its eastern side is singularly altered. It has here 
become a light frothy mass of cavernous quartz. Careful examination shows 
that this quartzite, though the main mass stands vertical, probably arched 
over to the eastward, and therefore forms a part of the anticlinal fold which 
adjoins, the fault on this side. The flat summit of the hill east of this point 
is made up of beds of White Limestone, included in which is a reddish 
decomposed porphyry. The actual curving of the White Limestone can 
scarcely be distinguished, inasmuch as decomposition has proceeded so far 
in the crest of the fold that a shallow ravine scores off the face of the hill 
adjoining the quartzite knob, in which all structural lines are obliterated by 
the sand resulting from that disintegration. Steep as are the north slopes 
here, it is useless to search for the actual fault line or the structure lines on 
either side of it. East of the fault there is no difficulty, and the Cambrian 
and Silurian beds overlying the Archean can be traced continuously along 
the wall of Mosquito gulch. Aside from the fact that the curve in the beds 
of this mass of white quartzite can be distinguished, its position adjoining 
the White Limestone would be sufficient to determine it as the Parting 
Quartzite, which forms the summit of the Silurian formation ; but in the 



several hundred feet of vertical beds which adjoin this on the west it would 
have been difficult, had no other opportunity for studying these faults 
offered, to determine satisfactorily whether they belong to the series on 
the eastern or those on the western side of the fault. Blue limestone 
and White Porphyry are here the former, it is true, represented only by 
a comparatively thin bed; and the other metamorphosed rocks might as 
well belong lithologically to the bottom as to the top of the Weber Grits 

The actual succession of vertical beds adjoining the quartzite crag on 
the west is, as well as could be determined, the following : 


Gap showing some black shale, about 40 

White Porphyry 20 

Bine limestone 8 

Quartzitic sandstones 100 

Blue limestone 8 

Quartzitic sandstones and decomposed greenish argilla- 
ceous beds, also silicious 200 

In the description given of faults it is generally stated that the flexing, 
occasioned by the movement of the faults, is reversed in the beds on either 
side. For instance, if the strata on the side of the fault that is lifted 
up are curved down by the dragging or friction of the movement, for the 
same reason those on the other side, which moves relatively downwards, 
would be curved upwards ; or if, on the other hand, on the upthrow side 
of the fault the strata are curved upwards as might be accounted for on 
the supposition of a force pushing from behind against the fault plane 
then the beds on the downthrow side of the fault are curved downwards. 
This is the generally accepted theoretical explanation of curving of beds 
adjoining a fault. In this case, however, we have the alternative of assum- 
ing that the beds curve downwards on both sides, or, what under the cir- 
cumstances is even more improbable, that a bed of limestone, which every- 
where else in the region examined has a thickness of 150 to 200 feet, has 
in this single locality been reduced to eight feet. It was assumed therefore, 
as shown on section D, that these vertical beds," as far as the quartzite crag, 
belong to the series west of the fault and geologically succeed the Weber 
Grits in regular order; that is, belong to the Upper Coal Measure horizon. 


The correctness of this assumption, so far as the horizon of the beds goes, 
has been proved by analogy in other localities, as will be described later, 
notably in the case of Weston fault on Empire Hill, where similar structural 
conditions exist, but with less intense alteration of the beds adjoining the 
fault, and where, moreover, the strata of the Upper Coal Measure formation 
were recognized definitely not only by their lithological characteristics but 
by abundant fossil remains found in them. The dividing line between the 
great silicious series of Weber Grits and the Upper Coal Measure formation 
having been arbitrarily assumed at the first development of calcareous 
beds, this line has been drawn on the map at the base of the lower bed of 
limestone mentioned in the above section. A thickness of something over 
one hundred and fifty feet of Upper Coal Measure beds is thus assumed to 
have escaped erosion on the western side of the fault. 

On the south wall of Pennsylvania Hill, facing Big Sacramento gulch, 
the beds which outcrop are practically identical with those on the north 
wall. They preserve the same strike of N. 20 W., with a dip of 20 to the 
east. The steepening of the clip as they approach the fault line is, however, 
not so apparent on the north wall of the hill, the surface being tq a still 
greater extent obscured by de"bris. Near the line of the fault the wall, as 
on the north side, is scored by a shallow ravine, on whose steep slopes frag- 
ments of White Porphyry are mingled with those of almost equally white 
quartzite. The former belongs evidently to the same body mentioned 
already as occurring on the north wall to the west of the assumed line of 
fault. Owing to the uncertainty which exists with regard to the structural 
relations of this body of White Porphyry, it has not been indicated either 
on the map or section. 

Sacramento arch. The south wall of Sacramento gulch, a sketch of which 
is given on Plate XV, presents an even more interesting study of the great 
London fault-fold than that of Pennsylvania Hill. The cliff section, as the 
sketch shows, presents a broad and rather flat arch, which has but little 
resemblance to the sharp S-fold already indicated on London Hill or to that 
which can be distinguished in the background of the sketch on the north face 
of Sheep Mountain. At first glance the curve on either side of the arch seems 
to be nearly equal in degree; but a more searching examination discloses on 


the right or west a few steep lines, indicating the nearly vertical dip of the 
beds adjoining the fault which is found at other points. A comparison of 
the direction of the valley with that of the axis of the fold affords a ready 
explanation of this deceptive appearance. The plane of the cliff section 
stands at an angle of 60 instead of at 90, or at right angles with the 
axis of the fold. So that nature has afforded a graphic illustration of the 
simple problem in descriptive geometry, the diagonal intersection of a 
cylindrical body by a plane. 

The interior of the arch is made up of Archean rocks, mostly gneiss 
with white vein-like bodies of pegmatite running through it. Over these 
stretch the entire lower Paleozoic series, with some interbedded porphyries, 
the principal of which is the Sacramento Porphyry in the Lower Quartzite, 
corresponding apparently in horizon with that on the north side of the 
gulch. Blue Limestone, more or less eroded, forms the crest of the hill. 
On the east side the beds slope away with the angle of the hill at about 
20. On the west of the crest, towards the fault, the dip rapidly steepens 
and becomes- vertical before reaching the fault plane. The structure can 
naturally be best seen on the cliff face. Here as elsewhere the stratified 
series seems much thinner in a vertical than when in a horizontal posi- 
tion. On the north face the Blue Limestone comes into contact with the 
fault instead of the Parting Quartzite, as on Pennsylvania Hill. The rock 
is much shattered and there is considerable development of black chert. 
Apparently some slight ore deposition has also taken place; but there is no 
evidence that this is the result of the faulting action. On the crest of the 
ridge, still east of the fault, are some shales and beds of impure anthracite, 
characteristic of the lower part of the Weber Grits formation. 

West of the Sacramento arch the ridge is level for a short distance, and 
then rises in a regular slope to the Gemini Peaks, two little projections 
crowning the ridge opposite the head of Sacramento amphitheater, on the 
north, and of Iowa amphitheater, on the west. The regularity of the struct- 
ure lines on the eastern flank or back of this ridge is extremely remark- 
able and is partially shown in the sketch. The dip of the beds, which to 
the west of the fault are entirely of the AVeber Grits formation, is here 
steeper than in the adjoining amphitheater, averaging from 25 to 35, 








and the lines of outcrop can be traced with the greatest distinctness. In 
tho distant view of the whole range from Mount Silverheels, as shown in 
Plate III, these structure lines, as well as the curves of Sacramento arch 
and of Sheep Mountain fold, can be readily recognized. Immediately west 
of the fault the beds are perpendicular, and even bend over so that they 
have a slight inclination to the westward. The change from this steep dip 
to the average inclination of the whole hill seems to be less sudden than on 
Pennsylvania Hill; but, as there, it is somewhat obscured. 

On the south side of the ridge facing Little Sacramento Valley is a 
slight synclinal fold, no evidence of which is found on the north face of the 
ridge. An explanation of this occurrence may be found in the fact that 
the line of fault from the Sacramento arch southward apparently diverges 
to the eastward, as compared with the strike of the beds, so that more space 
is left between the fault plane on the east and the unyielding masses of 
porphyry which form the crest of the ridge to the west. 

Gemini Peaks. In the long series of outcrops on the eastern slopes of 
the Gemini Peaks, which comprise almost the entire thickness of the Weber 
Grits formation, are some minor sheets of porphyry which have not been 
indicated on the map. The two peaks themselves form the crest of an 
immense body of Sacramento Porphyry which is exposed under the Weber 
Grits, both on the north and south walls, in apparent conformity with the 
overlying sandstones. The thickness shown, as derived from the angle of 
the slope, must be about 1,200 feet. The north branch of Little Sacra- 
mento Creek has cut to a great depth into this immense body of porphyry, 
leaving on either side walls nearly 1,000 feet in height, in which the same 
columnar structure in large masses or prevalence of vertical cleavage planes 
is found that has been already noticed in the porphyiy mass on the summit 
of Mount Lincoln. It evidently represents what was originally a huge lac- 
colite, and it is probable that it stands above the original vent from which 
the main flows of Sacramento Porphyry spread out into the adjoining rocks. 
Immediately to the south and west of this is the main body of White Por- 
phyiy, which forms the mass of White Ridge and of Mount Sherman. The 
junction of these two great bodies is extremely interesting, and was ex- 
pected to afford definite evidence of the relative age of the two rocks. The 


actual contact is, however, obscured by broken masses which almost invari- 
ably cover the- surface in these high regions. On the east side of the east- 
ern of the Gemini Peaks, however, were foi.nd a few beds of Weber Grits, 
within which was a small body of White Porphyry, while at either side of 
the Weber Grits was found Sacramento Porphyry. It seems, therefore, that 
this fragment of Weber Grits, with the included White Porphyry, was caught 
up within the later outflow of Sacramento Porphyry. Such caught-up 
masses of sedimentary rocks entirely included in porphyry masses are by 
no means uncommon. 

On the southwest face of the western of the Gemini Peaks are beds of 
Weber Shales, about fifty feet in thickness, consisting of gray limestone, 
quartzite, and green micaceous shales. About half a mile south of this, and 
in a shallow depression between the summit of Mount Sherman and the out- 
lying shoulder to the east, is a similar succession of beds, dipping however 
to .the west, which are entirely included in the surrounding mass of White 
Porphyry. On the east of this shoulder again, at the contact of Sacramento 
Porphyry and White Porphyry, are found thin beds of white quartzite, 
belonging undoubtedly to the same general horizon. 

The most characteristic exposures of this great mass of Sacramento 
Porphyry can be seen at the heads of Little and Big Sacramento gulches 
and on the main ridge between Sacramento and Evans amphitheaters. On 
the eastern wall of the latter it covers the greater part of its steep surface, 
widening and rising to the southward, and sleeping up to the summit of Dyer 
Mountain, where a thickness of some four hundred feet still remains. Below 
this, and separating it from the Blue Limestone, is a remnant of the lower 
beds of the Weber Grits formation, a relic of which forms the summit of 
West Dyer Mountain. From the saddle between Dyer Mountain and Gemini 
Peaks both Weber Grits and Sacramento Porphyry have been removed, 
leaving the crest of the ridge composed of White Porphyry. The limits of 
the two bodies of White Porphyry and Sacramento Porphyry are well 
defined by a line running nearly northwest and southeast between Gemini 
Peaks and Dyer Mountain. To the northeast of this line the White Por- 
phyry rapidly thins out and disappears. The occurrences of this rock, hith- 
erto noted in the regions farther north, were generally in the form of dikes 


of inconsiderable magnitude, or of quite small intrusive masses which doubt- 
less are the upper portion of similar dikes whose base is concealed. It is 
probable that these minor eruptions of porphyry are of later date than the 
main intrusive masses which prevail to the southwest of this imaginary line. 
Although the Sacramento Porphyry is not found upon the surface to the 
west of the main crest of the range, it is probable that it did not originally 
end abruptly there, but gradually thinned out in some such form as is indi- 
cated in Section D, west of the Mosquito fault, or as is shown more in detail 
in the sections accompanying the Leadville map. Lithologically it forms a 
definite type, whose general character has already been given in the chap- 
ter on Rock formations. Its distinguishing characteristics, as compared with 
the other porphyries, are its relatively large proportion of plagioclase feld- 
spar and its carrying hornblende. These ally it in some degree to the por- 

Little Sacramento gulch. The observations made in Little Sacramento 
gulch, which time did not admit of repeating, were unfortunately not suffi- 
ciently detailed to afford data for an accurate outlining of all the bodies of 
porphyry found there. The principal uncertainties resulting herefrom are: 
first, as to the eastern limit in the gulch of the main body of the Sacra- 
mento Porphyry : whether it confines itself to the horizon which it follows 
with apparent regularity farther north or whether it cuts across the over- 
lying beds ; and, secondly, whether a body of the same porphyry observed 
on the north face of the ridge separating Little Sacramento from Spring 
Valley is connected with the main body as a transverse body, or whether 
it is a portion of an interbedded sheet, like those on the western face of Lon- 
don Hill, with which it might be possibly connected by the bodies observed, 
but not outlined, on the eastern flanks of the Gemini Peaks ridge. 

Kast of the fault, it is evident that in the region included between 
Horseshoe and Big Sacramento gulches there is a lateral syncline similar 
to that observed on Pennsylvania and Lovelandllills, but broader and deeper. 
The sin-lace of the region is too much covered to admit of this fact being 
determined by the observed dip of the beds, but it is evident by the fact 
that the erosion of Little Sacramento gulch, where it lraver<e> the arch of 
tli- Sheep .Mountain fold, has cut down either to a very little depth or not 


at all into the Archean keystone of the arch ; whereas the erosion of the 
adjoining canons, Big Sacramento on the north and Horseshoe on the south, 
has cut into this body to the depth in one case of about five hundred and 
in the other of nearly one thousand feet. The sandstone of the Weber Grits 
formation overlying the Blue Limestone sweeps up on the ridges between 
Little and Big Sacramento gulches for a considerable distance above their 
junction, as is shown by numerous prospect holes. The continuity of the 
intervening belt of Sacramento Porphyry cannot be definitely proved, 
owing to considerable spaces where the outcrops are masked by surface 
accumulations, but is reasonably probable. 

Spring valley. The region between Little Sacramento and Horseshoe 
gulches is split by a little longitudinal valley, called Spring Valley, into two 
low ridges, either of which is capped by Blue Limestone. Their general 
form can be seen in outline on the Sacramento arch sketch, Plate XV. 

On the eastern slope of the northern of these two ridges is the Sacra- 
mento mine, which has obtained rich silver ores from the Blue Limestone. 
At the mine itself the overlying porphyry has been eroded off; but exten- 
sive outcrops, covering a very considerable superficial area, are found to the 
east, and are well shown in the steep rocky ravine which carries the drain- 
age of Spring Valley into the main Sacramento gulch. The same body of 
porphyry is found on the southern ridge, where it rapidly thins out, over- 
lapping a similar tongue of White Porphyry ; a portion of the Weber 
Grits formation is included between the two. It is evident that this body 
Of porphyry was once a continuation of the main body of Sacramento 
Porphyry, although it occupies a lower horizon and necessitates the supposi- 
tion that in separating out at a certain horizon a portion of the main lacco- 
lite body has cut down to a lower horizon. Improbable as this may seem, 
it can be pi'actically proved to have occurred on the south of the Twelve- 
Mile amphitheater, as shown in Section H, Atlas Sheet IX. Moreover, 
the thickest portion of this body is opposite the thickest portion of the main 

Horseshoe gulch. Perhaps the most complete and instructive series of 
sections, and certainly those which have the most direct bearing on the 
geology of the immediate vicinity of Leadville, are afforded by the erosion 


of Four-Mile or Horseshoe Creek. In regard to its nomenclature, local 
usage is somewhat perplexing. The stream itself, when it debouches on the 
South Park, is called Four-Mile Creek. Its main canon is generally known 
as Horseshoe gulch. At its head it divides into two branches ; to the north- 
ern of these has been given the name of Four-Mile amphitheater ; the south- 
ern branch heads in two adjoining cirques or amphitheaters, the northern 
of which has received, from its strikingly regular and complete curve, the 
name of the Horseshoe. (See Plate XVII.) 

This gulch, like those to the north, is glacier carved ; but the walls are 
less steep, as the upturned edges of the stratified rocks have been more sus- 
ceptible to subsequent abrasion, so that the talus slopes, covered with shrubs 
and trees, reach a considerable height. The wide gulch above the fault 
still has traces of lateral moraines along its sides. Where below the fault 
it is carved out of the Archean rocks, however, theee have been earned 
away by later erosion, the gulch being here considerably narrower. When 
the valley opens out again near East Leadville and bends to the southward, 
although there is moraine material on the lower slopes, the form of the 
ridges is not sufficiently distinct to show whether they are the original 
moraines or consist of rearranged material. On account of the importance 
of the district, the sections exposed will be described at considerable length. 
The appearance of the surface is shown in the accompanying sketches. 
That given on Plate XVI shows the more prominent outcrops on the ridge 
forming the north wall of Horseshoe gulch, from White Ridge, on the west, 
to the crest of the anticlinal fold, east of the London fault. 

white Ridge. The southwest face of White Ridge, as shown in the sec- 
tion, is a mass of White Porphyry. On its back and north and east slopes 
lie strata of Weber Grits formation, whose lines of outcrop can be traced 
as distinctly and regularly as those on the back of the Gemini Peaks Ridge. 
Their dip, however, is proportionately steeper, since the distance between 
the porphyry body and the line of fault is shorter. This dip, as shown 
in the sketch, varies from 30 to 45, the latter being the angle immediately 
above the White Porphyry, which to the eastward decreases gradually to 
30, and then, in close proximity to the fault line, rapidly steepens to the 
perpendicular. East of the fault the curves formed by the beds of the 


anticlinal fold over the Archean are very distinct, the partially eroded Blue 
Limestone forming the present crest of the ridge. 

On the south end of White Ridge, in the angle at the junction of Four- 
Mile amphitheater with the main gulch, is a prominent outcrop of dark-blue 
limestone, standing at an angle of 45, directly above the porphyry. A 
short distance to the west of this outcrop, at the foot of the steep slope 
and at intervals along the southwest side of White Ridge, on a line rising 
gradually as it approaches the head of Four-Mile amphitheater, prospectors 
with their keen natural instinct have traced the same bed under the heavy 
talus slopes of de'bris which cover it. 

In the very bottom of the Four-Mile amphitheater, as shown on the 
map, the Blue Limestone again outcrops in the bed of the gulch, and has 
been developed in the important Badger Boy mine and by numerous pros- 
pect holes. On the ridges around, White Porphyry forms the surface, which 
is in its normal position above the Blue Limestone. The line of the Blue 
Limestone, traced along the face of White Ridge, is however at a consid- 
erable distance above the actual level of the Badger Boy limestone, and at 
a still greater distance geologically, inasmuch as the normal dip is to the 
east, It is therefore evident that the limestone under White Ridge lias 
been lifted up by a fault, as shown in Section F, Atlas Sheet IX. The 
White Porphyry forming the mass of White Ridge is there in its normal 
position above the Blue Limestone, except at the south end just mentioned, 
where occur the prominent outcrops of dark limestone shown in the fore- 
ground of the sketch. The thickness of stratified beds exposed at this point 
is between 150 and 200 feet, the upper members of which have the character- 
istics of Blue Limestone, while toward the base are light-colored silicious 
beds, largely of white quartzite. Although the lithological character of the 
beds does not correspond in every respect with similar sections elsewhere, 
there is no doubt that it represents the main body of the Blue Limestone, and 
very probably the Parting Quartzite with a portion of the White Limestone 
beneath it. This heavy belt of dark limestone does not extend very far up 
the ridge, but gradually thins out and disappears, the sedimentary beds 
adjoining the porphyry at the summit of White Ridge being quartzites and 
micaceous shales of the Weber Grits series. It is evident, therefore, that 
the White Porphyry mass here cuts diagonally tip across the beds, and that 


White Ridge 


>' ' 
-^" ~-^ / 





Spring VaMey 

''. ,;.* 

4*\' *'''''''* 

* . ueologiat- in i 



the dark outcrop is simply a portion of the Blue Limestone left above it at 
this point, the main mass being represented by the line of Blue Limestone 
along the southwest base of the ridge. The thickness of the porphyry 
body, as represented by the distance between these outcrops, may be 
roughly estimated at about six hundred feet at the south end and 1,000 to 
1,500 under the summit of the ridge. It seems evident, therefore, that we 
have here in actual outcrop a portion of the main laccolitic mass as it ascended 
from below across the lower Paleozoic beds and spread out above the hori- 
zon of the Blue Limestone, as is shown theoretically in Section F. 

In the shales and quartzites on the northern and eastern slopes of White 
Ridge are numerous bodies of White Porphyry, which in the neighborhood 
of the summit sometimes seem to ramify and intersect the beds, but in 
general show a tendency to spread out between them. As it was impossi- 
ble to delineate all the varying outlines of these bodies, the prevailing form 
alone has been shown on the map, viz, that of intrusive sheets spreading 
out from the main laccolitic body along the stratification planes and grad- 
ually thinning as they depart from it. 

North wail of Horseshoe gulch. The section taken along the south face of the 
ridge eastward from the outcrop of Blue Limestone is approximately as fol- 
lows: A covered gap of about three hundred feet, containing, as is shown 
higher up, a bed of 50 feet of White Porphyry directly above the Blue 
Limestone; then about one hundred feet of shales, both calcareous and 
silicious, but mainly quartzite and sandstone; then a second bed of White 
Porphyry 50 feet in thickness, 5 feet of quartzite, and 5 feet more of White 
Porphyry; then varying quartzites, micaceous sandstones, and shales, above 
which are fine black shales, carrying pyrites and some fossils, from which 
were obtained the following forms: 

Producing seminticulatus. 
Productusmuricatus=P. longispinus Meek. 
Producing cora. 
Prodttctus costatus. 
Productus pertenuis. 
Griffith) des, sp. nutlet. 

Spirifcr cameratus. 

Spirifer, sp. ? 

A v icu lopecten carbon if cms. 

Fenestella, sp. undet. 

Rlwmbopora, sp. ? 

Fragments of crinoids and bryozoans. 

The above succession of beds, which is taken from notes by Professor 
Lakes, represents approximately what has been assumed as the Weber Shale 


division of the Weber Grits formation, viz, the fossiliferous and more calca- 
reous and argillaceous beds at its base. The thickness represented is some- 
what greater than that observed in other sections ; but the upper limits of 
the division are in themselves somewhat ill-defined, and the measurements 
obtained here are uncertain, owing to the fact that they were not observed 
in a continuous series of outcrops and certain beds may have been redupli- 

From here eastward to the fault the outcrops are those of the ordinary 
Weber Grits, coarse white sandstone predominating, with development of 
micaceous sandstones passing into shales, occasional thin seams of carbo- 
naceous shales, and a limited development of limestone beds Variation in 
the strike is noticed from N. 28 W., about midway in the series, to N. - f ) 
W., near the fault, The latter direction corresponds more nearly with the 
average strike of the beds near the fault, and the former may be considered 
to be a bowing out of the strata, caused by the intrusion of the large masses 
of porphyry at White Ridge and Gemini Peaks. 

The actual fault plane is apparently exposed by a prospect hole on the 
low saddle overlooking the gulch, where the contact of a dense quartzite, 
in vertical position, with White Porphyry on the east, shows very marked 
slickensides surfaces and a clay seam. A little to the west of this point is 
a second contact of quartzite and White Porphyry, dipping 50 east. This 
White Porphyry may very likely be an intrusion in the beds of the Upper 
Coal Measure formation, as has already been assumed to be the case with a 
corresponding body on Pennsylvania Hill. This assumption and the fact 
that the thickness deduced from the angle of the dip and the transverse dis- 
tance between this point and the base of the series necessitates the existence 
of a portion of the Upper Coal Measure beds, have been the reasons for their 
indication on the map and sections, since time did not admit of a suffi- 
cient!}" detailed examination to determine their existence on lithological and 
paleontological grounds. White Porphyry is found on the opposite side 
of the gulch, near the top of the Weber Grits formation, as will be shown 

Directly east of the fault, which occupies a saddle in the ridge, is a con- 


siderable outcrop of White Porphyry, whose thickness may be estimated 
"at 200 feet. Within the White Porphyry is a dark porphyry, very much 
altered, but similar in appearance to the Sacramento Porphyry, and which 
may once have been connected -with the body of this rock already described 
above the Sacramento mine. These are succeeded by the Blue Limestone, 
whose beds, as shown in the section and sketch, curve up and cover, some- 
what irregularly, the double-pointed ridge over the arch of the fold. From 
this limestone well-preserved specimens of Spirifera Rockymontana were 
obtained. In the Blue Limestone on the crest of the arch are, according to 
Professor Lakes, numerous vertical cracks, which may be cross fractures 
resulting from folding. The lithological character of the Blue Limestone 
varies greatly in different portions. Black chert concretions, which are as 
elsewhere most frequent at its summit, are also found well down in the for- 
mation. Many of the beds, especially near the base, are comparatively 
light-colored. No satisfactory continuous section was obtained of the lower 
Paleozoic beds, though the estimate of their aggregate thickness does not 
vary from that obtained elsewhere. At various points an included bed oi 
White Porphyry, near the top of the Lower Quartzite, and averaging about 
thirty feet in thickness, was observed. The Archean is composed of gneiss, 
and of red porphyritic granite with large orthoclase crystals. 

On the eastern slope of the anticline, outcrops of beds above the Bliu 
Limestone are exposed in the forest-covered region near the road leading 
from East Leadville to Spring Valley, where they are much obscured by 
surface accumulations, and, on the steeper slopes, by the relics of a lateral 
moraine. Above the Blue Limestone the White Porphyry can first be distin- 
guished; next is an interval of coarse sandstone; then a body of Sacra- 
mento Porphyry, which apparently thins out rapidly to the southward. 
The White Porphyry, on the other hand, rapidly thickens in that direction, 
as shown by its section on the eastern slope of Sheep Mountain. 

An attempt was made by Professor Lakes to obtain a continuous section 
from here eastward, through Fairplay, across the upper members of the 
Carboniferous and the overlying Triassic, Jurassic, and Cretaceous beds. 
The result was not very satisfactory, inasmuch as a great portion of the 


line of section is occupied by covered gaps, which could not be accurately 
filled by offsets. The thickness of the sedimentary series from the Cam- 
brian -up to the top of the Cretaceous along this line has therefore been 
assumed, in the ideal reconstruction of the surface, as that given by the sec- 
tion of the Hayden Atlas, viz, 10,000 feet. The data obtained by Pro- 
fessor Lakes afford no sufficient reasons for differing from this general con- 

Four-Mile amphitheater. The description now turns to the exposures at 
the head of the gulch and along the main crest of the range from Mount 
Sherman to the head of Twelve-Mile Creek. The most striking of these 
are shown in the sketch on Plate XVII, which represents the Horseshoe 
and a portion of the Four-Mile amphitheater as seen from the junction of 
the two branches of the creek. The shapes of these two amphitheaters dif- 
fer characteristically, in accordance with the differing characters of the rock 
out of which they have been carved. The erosion of the Four-Mile am- 
phitheater, which has been practically parallel with the strike of the beds, 
has acted almost exclusively on the great mass of White Porphyry. Its 
slopes are generally more rounded and largely composed of talus slopes of 
angular fragments of this geologically brittle rock. In the bed of the stream 
erosion has denuded a narrow strip of the Blue Limestone, dipping 16 to 
the N-. E. and striking N. 15 to 20 W. East of this outcrop a bed of Blue 
Limestone, as already mentioned, has been developed by a line of prospect 
holes along the face of White Ridge, whose elevation to its present relatively 
higher position must necessarily have been the result of faulting. Data are 
wanting, however, to locate definitely the line of this fault. That given on 
the map as the Sherman fault is determined principally from the theoretical 
considerations furnished by Section F, according to which it is assumed that 
a certain arbitrary thickness of White Porphyry exists under the Blue Lime- 
stone. The fault line would therefore have White Porphyry on either side 
of it, which necessarily renders its position difficult to recognize. That such 
a body does exist under the Blue Limestone is rendered almost certain by 
the fact that it is found at this horizon farther westward, along the western 
base of Mount Sheridan and throughout the Leadville region to the north- 
east of a line roughly drawn from Mount Sheridan to Fryer Hill. 


Horseshoe M 


- ^> \ -*, 

^ / ^* ; 

' // t 




s.I-' Kmmons, Geologist m Cli;uC<- 



Besides these two outcrops of limestone the only sedimentary beds ob- 
served are a lenticular body of Weber Grits at the head of the amphithe- 
ater on the south face of Mount Sherman. This body, which is several 
hundred feet in length and thirty or forty feet in thickness, consists of shales 
and sandstones, the former apparently somewhat baked and the latter 
changed to quartzite. It extends to within a few feet of the top of the divid- 
ing ridge between Four-Mile and Iowa amphitheaters, but does not outcrop 
on the wall of the latter. 

The western slope of Mount Sherman, which forms the eastern wall of 
the Iowa amphitheater and is shown in the background of the frontispiece 
of this volume, consists, from the crest two-thirds way down, of a mass of 
White Porphyry from 1,200 to 1,500 feet thick. Separating this from the 
Archean in the bottom of the gulch are the lower Paleozoic series, whose 
beds rise to the southward as one follows the wall and curve round the 
west face of Mount Sheridan across the low saddle which separates it from 
West Sheridan. The sharp crest of Mount Sheridan and its eastern slope 
are covered with White Porphyry, as is also the little eminence south of it 
on the main ridge, called Peerless Mountain. On the saddle between the 
two the White Porphyry has been eroded off for a considerable distance 
down the east slope, and certain rather silicious beds i esembling quartzite, 
which here form the upper portion of the Blue Limestone, have been ex- 
posed. South of Peerless Mountain the Blue Limestone is again exposed 
on the surface of the crest, as far as the top of Horseshoe Mountain, and 
also in a strip bordering the Horseshoe on the northeast. In this vicinity, 
especially along the western face of Peerless Mountain, the upper portion 
of the Blue Limestone shows evidence of considerable inetamorphic action. 
Its outcrops are quite dark, and its upper part, as already mentioned, is very 
silicious and resembles quartzite. It has also a slightly brecciated struct- 
ure, and in certain places is very much stained with oxides of iron and man- 
ganese. It is probable that this alteration is due to mineral waters, and is a 
commencement of decomposition such as has gone on in Leadville itself, 
though the amount of lead and silver ore as yet developed is comparatively 
inconsiderable. The darker color is due doubtless to oxide of manganese, 
and the silicification of the beds to percolating waters depositing granular 


silica, a form of vein material which, as will be seen later, is common in 
the Leadville mines and easily to be mistaken for genuine quartzite. The 
brecciation is doubtless due to the action of the porphyry at the time of its 

The Horseshoe. Horseshoe Mountain, as is shown both on the map and 
on the sketch, is covered by a thin shell of easterly-dipping- beds of the 
lower Paleozoic series, whose angle on the crest is about 10 and steepens 
to an average of 20 on the eastern slopes. The irregularity of the out- 
crops of the successive formations shown on the map represents the results 
of erosion on this thin shell. 

The character of the outcrops in the Horseshoe itself is sufficiently 
shown in the sketch. Its peculiar form is a result of glacial erosion, which 
alone could have carved vertically across the inclined surfaces of hard sedi- 
mentary strata. The main body of the encircling cliffs is composed of the 
White Limestone and of the upper beds of the Lower Quartzite, which, ow- 
ing to their peculiar weathering, received in the field the convenient name 
of "sandy limestones." On their weathered surface they resemble in all 
respects a sandstone, but a fracture of the mass shows the interior to have 
the compact semi-crystalline structure of limestone. The beds of Blue 
Limestone above these are more or less eroded off, while the pure quartz- 
ites at the base of the series are in places concealed under the talus 
slope of debris. In the very bottom of the amphitheater are two or three 
little shallow lakes or ponds of glacial origin, carved out of granite or tho 
Lower Quartzite. Passing down the stream from the glacial amphitheater, 
one crosses successively an ascending series of outcrops which sweep round 
in graceful curves up the bounding ridge to join the beds on the crest of 
the range. 

Intersecting these outcrops in a northeasterly direction, and in part 
following the line of contact between the Blue and White Limestones, is 
a small body of porphyrite; this and a similar outcrop in the Four-Mile 
Amphitheater constitute the only instances observed of the occurrence of 
this rock within the White Porphyry region. The rock is a grayish-brown, 
homogeneous-looking, fine-grained mass, showing small glistening black 
biotites and minute white feldspar crystals with round quartz grains. Under 


the microscope there seems to be no fresh feldspar substance left in the 
rock, although outlines of former crystals can often be plainly distinguished, 
the interior being replaced by a mixture of calcite and a cryptocrystalline 
substance, colorless in ordinary light, showing the alternations of light and 
dark points characteristic of a homogeneous aggregation of minute particles, 
probably quartz. The biotite leaves, both large and small, seem perfectly 
fresh and in remarkable contrast to the condition of the feldspar. From the 
great quantity of calcite present and the absence of muscovite or kaolin, it 
seems evident that the feldspar was a plagioclase rich in lime, and the rock 
a quartz-biotite-porphyrite, although in external appearance it is quite unlike 
any porphyrite observed elsewhere in the region. 

The larger amphitheater at the head of the south fork of Horseshoe 
Creek has a less striking and regular form than the Horseshoe itself, but 
presents the same geological structure. From the crest of the range at its 
head, however, the Blue Limestone has been eroded off, and Silurian beds 
form the surface. These are succeeded, as one goes south along the crest 
to the head of Twelve-Mile amphitheater, by the Cambrian and Archean 

South wail of Horseshoe gulch. On the ridge running from the crest of the 
range to Sheep Mountain, along the south side of Horseshoe gulch, an excel- 
lent continuous series of beds from the Archean up to near the top of the 
Weber Grits are shown. The north side of this ridge is most admirably 
delineated by a line sketch from the skillful hand of Mr. W. H. Holmes in 
the Hayden report for 1873. l 

The same series of beds are here represented as were shown on the 
ridge north of the gulch, but they occupy nearly double the space in lineal 
extent along the side of the gulch; their angle of dip is consequently 
shallower, and midway in the series is a small synclinal fold which enables 
the same beds to cover a greater surface. The direct connection between 
the two sides is obscured by Ihe detrital material in the gulch. It is evident, 
however, that the existence of a cross-fault is necessary to explain this dis- 
crepancy, since there is no evidence that the beds of the south ridge curve 
round to the east to join those on the north, their strike being the normal 

'Page 230, Geological and Geographical Survey of the Territories. Washington, 1874. 


strike of the formations, N. 10 to 20 W. This fault has been assumed, 
therefore, to follow the bed of the gulch, and probably connects the Sher- 
man with the London fault ; its line is not given on the map, as it would be 
concealed by the Quaternary beds indicated in the bed of the gulch. The 
course of the gulch in this extent, which is unusually straight, has probably 
been determined by this fault. 

The structure of this Sheep Mountain ridge, as deduced from careful 
observations made along its surface, is shown in Section G, Atlas Sheet 
IX. Of the White Limestone and Lower Quartette, which are only exposed 
in the amphitheater south of the Horseshoe, measurements were not made, 
since those obtained from the exposures in the Horseshoe itself correspond 
with the thickness obtained elsewhere. The body of White Porphyry, which 
sweeps up at an angle of 20 opposite the opening of the amphitheater, has 
here a thickness of nearly two hundred feet, and shows a certain tendency to 
columnar structure at right angles to the bedding. The beds immediately 
above the White Porphyry contain a large proportion of shales, which, being 
easily disintegrated, show but few outcrops, the space occupied by them 
forming a saddle in the ridge. The thickness from the White Porphyry 
up to the more persistent sandstones and grits of the Weber series, which 
would correspond to the Weber Shale division, is here estimated at from 
two hundred to three hundred feet. The beds observed are as follows : 
Directly above the White Porphyry is a bed of black carbonaceous shales ; 
from one hundred to one hundred and fifty feet above it is an outcrop of 
dark, impure limestone, from which were obtained a large number of fossils, 
among which the following were recognized: 

Chonetes yranulifera. 

Productus corn. 

Productus nodosun (variety of Pro- 

(luctus cora). 
Product tin sem iretic ula tus. 

Fragment of Pinna, sp. 
Fragment of Aviculopecten. 
Phillipsia, sp. 
PhillipalH major. 
Fragment of Lingula, sp. 

Myulina pcrattenuata. 

About fifty feet above this there is a bed of black shales, from which 
were obtained impressions of Lingula mytiloules, the same form which is 
so abundant directly above the Blue Limestone near Leadville. For 
about three-fourths of a mile eastward along the crest of the ridge the 


beds dip regularly eastward at an angle of 20. They consist mainly 
of coarse white or gray sandstones, passing into conglomerates composed 
largely of pebbles of white milky quartz, having a slightly pinkish tinge, and 
which, when weathered out, cover the surface for a great distance. Alter- 
nating with these are thinner beds of micaceous quartzite, passing into a 
mica-schist, the mica being always of the muscovite or potash type. Less 
frequent are thin seams of black carbonaceous shales. Near the upper part 
of this portion of the section is a single bed two feet thick of dark iron- 
stained limestone, seamed with carbonaceous shales. Between this and a 
little knob rising above the general level of the ridge is a synclinal fold in 
the beds, which rise on its western side at angles of from 50 to 70. 
The beds included in the synclinal trough above the iron-stained bed are, 
first, a white quartzite conglomerate, then a brownish sandstone, then a 
white massive sandstone, then a second brownish sandstone with thin seams 
of clay and shale, and finally a green clay slate at the axis of the syncline. 
East of this axis the same succession of beds is passed over, which appear 
thinner, however, owing .to their standing at a steeper angle. On the east 
side of the knob the iron-stained limestone reappears dipping 70 to the west, 
and a short distance farther can be traced somewhat indistinctly, dipping 
at the same angle to the eastward. Following the ridge eastward the beds 
assume the normal dip of 20 and have the same general character as that 
already described. For half a mile or more dark thin beds of quartzite and 
shaly beds are more frequent, but gradually pass up into massive, heavily- 
bedded, coarse white sandstones, whose dip shallows to about 10 or 15. 
This little anticlinal and synclinal fold has the normal character of the folds 
in this region, viz, a steep west side to the anticline or east side to the syn- 
cline. It may also, as is often the case, be accompanied by a slight move- 
ment of displacement, but this could not be definitely proved. The synclinal 
structure can be traced on the broad ridge directly south of this point in the 
somewhat indistinct lines on its grassy surface which mark the outcrops of 
the beds. The fold here becomes broader and shallower, and probably soon 
dies out to the south. 

Lamb Mountain. Near the west end of the little prominence on the ridge 
called Lamb Mountain, an eruptive rock comes in above the sandstone, 


which weathers in large shaly blocks, with a remarkably beautiful con- 
choidal fracture and the peculiar sherdy habit which is common among 
volcanic rocks. The rock is white, slightly tinged with reddish yellow, due 
to minutely disseminated particles of hydrated oxide of iron. In the fresh 
fracture it shows a white granular homogeneous mass, with occasional grains 
of feldspar. It was first thought to be a later eruptive rock, probably a 
rhyolite, but careful microscopical study shows it to be a true White Por- 
phyry, differing in no essential from the normal type. On Lamb Mountain, 
as shown in the sketch, Plate XVIII, this body has a maximum thickness 
of about four hundred feet at the summit of the hill, its lower limit corre- 
sponding in general with the bedding plane of the underlying sandstone. 
This correspondence, however, on close examination, is not absolute, inas- 
much as it occupies a slightly lower horizon to the eastward, and on the 
north face of the ridge just west of the ravine between Lamb and Sheep 
Mountains it can be seen to cross the beds nearly at right angles, in the 
form of a dike. On the steep east side of Lamb Mountain toward the 
saddle are beds of slate and micaceous sandstone, curving up at an angle of 
50 against the eruptive mass. In these slates were found abundant im- 
pressions of Equisetce, or Horsetails, a plant characteristic of the Coal Meas- 
ures. Sandstone outcrops can be traced on this saddle and across it to the 
base of the steep western slope of Sheep Mountain, where they soon dis- 
appear beneath the plentiful ddbris of White Porphyry. The White Por- 
phyry from which they come is, as will be shown later, the body which 
belongs above the Blue Limestone; therefore the fault line must run very 
nearly at the foot of this steep western slope. That the Lamb Mountain 
body is itself a small laccolite, with a separate vent or channel, is evident 
from the fact that it ends abruptly on the east and that, while the steeply- 
dipping beds rest against it on the saddle east of the peak, lower down the 
slope of the hill the dike-like channel, which extends downward from the 
main mass of porphyry, is found to cross the shallow-dipping sandstone strata 
without perceptibly changing their angle. The steepness of the beds on the 
saddle might be explained by the expansion of the intrusive body of por- 
phyrv, which would push them up, but this explanation is rendered unnec* 
essary, since, as we have already seen, the beds immediately adjoining the 


* A * *- i A *T* ft M^-'^Jf- * 


. ' i , xvni 


ft . ,->* 

; y 



OkV, ,e 


fault on the west are always found to stand at a nearly vertical angle. The 
Lamb Mountain laccolite, it must be borne in mind, is at a higher horizon 
than the main sheet of White Porphyry. It may be an irregular offshoot 
from the White Ridge laccolite, or, as shown in Section Gr, simply an extru- 
sion from the main sheet. 

sheep Mountain. Sheep Mountain itself is an important peak, having an 
elevation of over two thousand feet above the bed of the gulch and form- 
ing the northern culmination of a ridge running in a nearly northwest and 
southeast direction, whose form is closely connected with its geological 
structure, since the line of fault south of Sheep Mountain follows approxi- 
mately its crest. The internal structure of the peak is best exposed on the 
north face, a view of which is shown in Plate XVIII. The eastern slope 
of Sheep Mountain is a little less steep than the dip of the beds, for which 
reason the White Porphyry which crowns its summit is denuded over a con- 
siderable portion of the slope, and comes in again at the foot, where the 
slope becomes more gentle. The western side of the fold, as shown in the 
sketch, is very nearly vertical In point of fact, however, the angle is a 
little over the vertical, or, in other words, the beds dip slightly east, as 
shown in section G. This is not apparent, however, on the cliff, from the 
fact that its plane is not exactly at right angles with the axis of the fold. 
Here, again, as in the Sacramento arch, the series of beds when vertical 
appear thinner than when standing nearly horizontal ; in other words, they 
seem to be compressed between the arch of the fold and the plane of the 
fault, which is not at all impossible or even improbable. Unfortunately, it 
could not be determined by actual measurement, as there was no continu- 
ous outcrop of the vertical beds. 

A section was carefully made across the beds from the crest of the fold 
to the summit of the peak by Mr. Cross, from whose notes most of the fol- 
lowing data are taken. The Archean exposures are mainly of gneiss and 
their bedding is comparatively distinct. As well as could be ascertained, the 
beds stand nearly vertical and have an east and west strike, or at right 
angles to the axis of the fold. Adjoining the vertical Cambrian beds was 
noticed a little irregular dike of White Porphyry about four feet in thick- 
ness, which comes out of the gneiss nearly pai-allel to the strike of the 


quartzite and then cuts obliquely into the latter for a short distance; it, then 
follows the bedding-plane for a few yards, and, again cutting across the 
strata, disappears under the debris. The measurements were made with a 
pocket level, checked by observations with an aneroid barometer, taken at 
the base and again at the summit of the cliff; the discrepancy between the 
two measurements amounting to only a few feet. 

Section from top of Sheep Mountain downward: 


White Porphyry, 300 to 400 feet. 

!Blue Limestone, brecciated at top, with abundant 
secretions of black chert. 180 
Lighter-colored limestone 20 

{Parting Qnartzite, line Drained, white 70 
White Limestone, silicious at base, with white chert 
secretions 100 

f Ued cast beds 8 

I Shales, interbedded with "sandy "limestones" .'!0 

Reddish, fine-grained sandstones, with indistinct im- 

Cambriau.. ..\ Passions 40 

Gap i 10 

Qnattzite 22 

White Porphyry, 12 feet. 

White contact quartzite .... 65 



Archean G neiss 

The total thickness here obtained of the lower Paleozoic series, which is 
605 feet, is a little greater than that obtained at other points, which may possi- 
bly be due to the swelling of the beds that would naturally succeed a com- 
pression, if such exists, on the side of the fold next the fault. The contact 
of the White Porphyry and underlying Blue Limestone, which was here 
visible over a considerable distance, was carefully studied, especially on the 
side of the fold next the fault. The upper part of the Blue Limestone is 
particularly dark and full of black chert. The actual line of contact is 
marked by a breccia, whose character varies much. Now it is composed 
mainly of White Porphyry fragments, then of chert, and again of a mixt- 
ure of both with black shale or limestone. Sometimes arms of the White 


Porphyry penetrate the mass of limestone. On the steep southwestern 
slope of Sheep Mountain, overlooking Sheep Park, the brecciated surface 
of the Blue Limestone projects through the White Porphyry. A curious 
feature of this breccia is the character of its cement, which is crystallized 
gypsum and quite abundant here, though not noticed elsewhere. The exist- 
ence of so much breccia at this point would strike the observer on first 
view as probably due to the action of folding and the friction occasioned 
by that and the displacement of the fault. Inasmuch as the same phenom- 
ena are observed, although on a lesser scale, at the contact in Leadville, 
where the folding action has been comparatively slight, it is probable that 
it was induced by a fracture of the more brittle portion of the surface of 
the limestone in contact with the molten intrusive mass at the time of its 
eruption. The fragments of porphyry in the breccia do not necessarily 
militate against the supposition, since the shell in immediate contact with 
the bounding beds might cool and harden and then be broken up by a 
fresh body of molten porphyry pushing over it. The gypsum cement is 
an evidence of the passage of sulphurous waters, which would form sul- 
phate of lime by their contact with the underlying limestone, depositing it 
again in the crevices of the fragments on the surface. That it still remains 
here seems to be an evidence that the dissolving action of later waters has 
not been continued so long as in Leadville, where almost every trace of 
gypsum hai been carried away. 

On the southeast slope of Sheep Mountain, near the timber-line, are 
several rounded foot-hills, between which the White Porphyry and the Blue 
Limestone are exposed in the ravines, while the Weber sandstones form the 
surfaces of the intervening ridges. A number of prospect tunnels have 
been run in these sandstones, disclosing irregular shale formations in the 
beds and a local development of White Porphyry above the regular body. 
In one of the tunnels the end of this intrusive body is well seen, showing 
the beds curving around it, as in the intrusive mass of porphyrite on South 
Mosquito section. The average strike of the beds here is from N. to N. 10 
W., and the dip from 27 to 30 eastward. 

As this locality presents the most typical development of White Por- 
phyry outside of the immediate vicinity of Leadville it may be well to 


describe somewhat in detail the rock as found here. That from the west 
face of Lamb Mountain (46), which is comparatively fresh, is a compact 
rock, of a light pinkish-brown color, whose only visible crystals are a few 
small and well-defined orthoclase individuals. No quartz is to be seen. 
Minute cavities lined with yellow ocher indicate a former constituent, but 
the forms of the cavity are not sufficiently well preserved to indicate it* 
character. It may have been pyrite. Under the microscope the rock 
appears granular, with easily determinable quartz, orthoclase, plagioclase,. 
and muscovite. There is no trace of a microscopic groundmass between 
the grains. Both orthoclase and plagioclase are abundant, but muscovite 
is less developed than is usually the case in White Porphyry; contrary 
to the usual rule, it is as often found forming in plagioclase as in ortho- 
clase. With a low power, the feldspars seem full of fine dust or specks, 
which in many cases are evidently arranged on the cleavage plane. These 
specks are also seen, though less frequently, in the quartz. By the use of 
a higher power it is seen that some of these specks are fluid inclusions, 
with rapidly moving bubbles, and it is therefore probable that a sufficiently 
high power would prove that all are similar inclusions. No glass inclu- 
sions were found. The main rock on the northwest slope of Sheep 
Mountain (47) is of porphyritic appearance, owing to the large devel- 
opment of muscovite; otherwise it does not differ microscopically from the 
Lamb Mountain rock. A contact specimen of this body is so fine grained 
that its exact composition cannot be made out, yet it does not seem to differ 
essentially from the average rock. Portions of the body are perfectly white 
and homogeneous, and when breathed on have a strong earthy smell The 
specimens examined contained little, if any, plagioclase and almost no mus- 
covite. The body included in the Weber Grits formation (49) is exactly 
the same as the ordinary rock. That from the saddle between Sheep and 
Lamb Mountains contains even more plagioclase than the Lamb Mount- 
ain type. That from White Ridge (44), in the Four-Mile amphitheater, is 
extremely white and very compact, so that the constituents are mostly 
indistinguishable. The process of alteration from feldspar to muscovite 
can readily be distinguished by the naked eye. 



The remaining portion of the eastern slopes of the range included in 
the map, south of Horseshoe gulch, presents but few good exposures as com- 
pared with the region already described. Its altitude is generally lower 
and the surface is covered with forest growth and very considerable accu- 
mulations of Quaternary gravels; still, the general outline of its structure 
is not difficult to seize. 

sheep Ridge From Sheep Mountain to Round Hill the crest of this 
ridge becomes gradually lower, and beyond the latter it disappears under 
the plain. Immediately south of the summit of Sheep Mountain is a slight 
depression, from which the White Porphyry has been eroded off, exposing 
the underlying Blue Limestone. Again, at the first prominent saddle in the 
ridge, Blue Limestone forms the crest and "the eastern slopes, and beyond 
this to Warm Spring pass White Limestone outcrops along the crest, show- 
ing that the topographical slope descends more rapidly than the geological. 
At Warm Spring pass fragments of Red-cast beds on the crest indicate that 
the whole thickness of the Silurian probably comes to the surface here, 
although no actual oittcrops of Cambrian beds could be detected. The 
steep western slopes of the ridge toward Sheep Creek, in this extent, are 
formed of. easterly-dipping Weber Grits, and are therefore on the western 
side of the London fault. On the eastern slope the White Porphyry sheet 
appears to be continuous above the Blue Limestone, and at the Warm 
Spring pass has thinned out to 20 feet. The so-called Warm Spring fur- 
nishes a considerable flow of water, of a temperature of about 60, from the 
upturned strata near the base of the Blue Limestone. South of Warm 
Spring pass, judging from the meager data afforded by outcrops, the geo- 
logical slope becomes greater than the topographical. The Blue Limestone 
forms a cliff half way up the slope on the south side of the pass, and beyond 
this the only rocks found on the surface are those belonging to the Weber 
Grits ; these on Round Hill have an anticlinal structure, dipping to the 
east, south, and west, although along its extreme western face an eastern 
dip is again found, which is the commencement of the slope of the beds 
upwards toward the crest of the main range. The explanation of the 
structure of this portion of the hill is that the fault movement has died out 
and only the fold remains. 


The surface of the South Park, east of Sheep Ridge, is uniformly 
covered to a considerable depth by Quaternary gravels, the only outcrop 
of underlying beds within the limits of the map being north of the bend 
of Four-Mile Creek, where an anticline in the Weber Grits can be seen, a 
continuation of the secondary roll already noticed to the north. East of 
the limits of the map the approximate location of the Triassic beds is indi- 
cated by the red color of the soil, and the more resisting beds of this and 
the higher formations form low north and south ridges, which rib the surface 
of the park. With these are associated sheets of eruptive rock, probably 
analogous to the intrusive 'sheets already described. On one of these ridges 
was found the rhyolite tufa which is described in Appendix A. 

Black Hill. Out of the South Park plain, at the extreme southeast corner 
of the map, rises to a height ef about GOO feet an isolated, forest-covered 
hill, nearly circular in shape, known as Black Hill, only the northern edge 
of which comes within the limits of the map. It is entirely composed of 
rhyolite (140). The occurrence is interesting on account of the rarity of 
Tertiary eruptive rocks in the region under consideration. It is noticeable 
that it is on a direct line with the continuation of the London fault, and 
that the prolongation of the same fault to the northwest would nearly pass 
through the other occurrences of rhyolite in Chalk Mountain, on the north- 
ern edge of the map. The whole mass of the hill is composed of rhyolite, 
as far as can be distinguished. The outflow has apparently taken place 
through the upturned sedimentary beds and spread out over their edges, 
without, however, exercising any very marked influence on their structure 
lines, as is the case with the secondary intrusive masses. The outcrops of 
these sedimentary beds are somewhat obscure, being mostly covered by sur- 
face accumulations, but, from their lithological character and from the succes- 
sion observed along the valley of the Little Platte, south of the limits of the 
map, they are assumed to belong to the horizon of the Upper Coal Measure 
formation. On the northern base of the hill is a considerable accumulation 
of impure gypsum in mud shales. Directly south of the hill, along the basin 
of the Little Platte, quite a succession of thin-bedded clay shales, with some 
limestone beds, is found, standing nearly vertical and striking due north 
and south. In a prospect hole these shales are seen to be remarkably con- 


torted, which is probably due to the original compression of the beds, and not 
dependent on the outflow of rhy olite. The lines of strike, so far as observed, 
run continuously through the hill, and do not curve round it. Almost 
the whole surface of the hill is covered with loose fragments, detached 
through frost and atmospheric action, but its south and southeast faces pre- 
sent steep cliffs. On the lower northeastern slopes of the hill are two or 
three large bowlders of coarse reddish granite, half buried in the soil, in 
company with quartzites and sandstones, which are evidently erratics and 
. show that at one time the glacier from Twelve-Mile Creek reached down as 
far as this. 

The rhyolite of Black Hill is remarkably uniform in general character. 
It has a delicate pinkish-gray color, a conchoidal fracture, and shows in the 
unaltered specimen white glassy feldspars, fresh black mica, and some horn- 
blende, with prominent and rather smoky quartz in a distinctly marked 
groundmass. The existence of this groundmass makes a marked distinction 
from the rhyolite of Chalk Mountain, which is seenmacroscopically to be made 
up entirely of crystalline elements. To the naked eye it is apparent that 
the quartz contains many bays of the groundmass. Under the microscope 
the groundmass is seen to be entirely microcrystalline, being composed 
mainly of quartz, with some rather cloudy feldspars. The large feldspars are 
plagioclase in part and contain a few gas pores and some fluid inclusions, 
which often carry cubes of a mineral like salt. Undoubted glass inclusions 
are not visible, but there are some dihexagonal in form, which are either 
devitrified inclusions or represent the character of the groundmass at a time 
prior to its complete crystallization. In decomposition the feldspars seem 
to tend more to a kaolin substance than to nmscovite. 

Twelve-Mile Creek. In the region between Sheep Ridge and the main crest 
of the range are the valleys of Twelve-Mile and Sheep Creeks. The sur- 
face is covered with outcrops of Weber Grits formation, or, in its lower por- 
tion, with surface gravels, either actual moraines or rearranged drift mate- 
rial. At the head of Sheep Creek, near the south base of Lamb and Sheep 
Mountains, is a little valley or park, bounded on the north and east by steep 
talus slopes of debris from these peaks, and by forest-covered spurs of 
Weber. Grits on the south and west. On the broad ridge between Horse- 


shoe and Twelve-Mile Creeks the shallow syncline in the Weber Grits, 
already mentioned, can be traced as far as the Twelve-Mile gulch. It 
is only in the deeper cuts near the crest of the ridge that the details of 
structure are distinctly visible. Twelve- Mile Creek heads in four sepa- 
rate basins or amphitheaters, to the distinctness and grandeur of whose 
forms the scale of the map can do but scant justice. The exposures of 
Archean rocks in these amphitheaters present a great variety of gneiss and 
granite, the most noticeable of which have already been described in Chap- 
ter III. The deeper of these amphitheaters is that to the north, whose north- 
ern wall is capped by beds of the lower Paleozoic series, the Lower Quartzite 
forming, as shown on the map, the crest of the range at its head. The sheet 
of White Porphyry above the Blue Limestone has a broad outcrop, prom- 
inent by its white color, extending across from Horseshoe Ridge and sweep- 
ing down the wall across the mouth of the amphitheater. On the ridge 
between this north amphitheater and the one adjoining it, a shell of Lower 
Quartzite still remains at its eastern end. South of this, Paleozoic out- 
crops are confined to the meadows at the lower extremity of the amphi- 
theater, where a number of springs come from them. On the south wall of 
the southern amphitheater the lower Paleozoic beds again sweep up for a 
considerable distance on the spur, the white quartzite of the Cambrian and 
the interbedded White Porphyry being prominent by their color. The 
eastern end of this ridge is formed by the continuation of the main White 
Porphyry body; while along its wall can be traced an. offshoot from this 
body, cutting across the Blue Limestone and occupying the horizon between 
the Blue and the White Limestone. The white quartzite extends nearly up 
to the prominent shoulder of this spur, and is found again on the very summit 
of Weston's Peak, at the head of the spur. Here it lies nearly horizontal, 
bending over slightly on its western edge. This mass of quartzite is evi- 
dently, as shown in Section H, a remnant of the crest of the anticlinal fold, 
whose axis relatively to the present slope of the ground is descending to the 
southward. The outcrops of the sedimentary beds on the east of the axis, 
therefore, gradually rise along the eastern slopes of the ridge; their outlines 
as shown on the map present- a series of regular curves, due to the erosion 
of the ravines which score the surface, which are distinguishable in the 



field from a considerable distance through the whiteness of the quartzite 
and of the interbedded White Porphyry. The anticlinal fold of the main 
crest, like that of Sheep Ridge, gradually 'dies out to the south of the map, 
and at Buffalo Peaks has entirely disappeared, being merged into a single 
monoclinal continuation of that to be described on South Peak. 

Western's pass. On the steep western face of the crest, towards the 
valley of the Little Platte below Weston's pass, the Lower Quartzite and 
White Limestone beds lie at an angle of 45 to 50, resting against the 
steep slope of the hill like tiles on a roof. The valley of the Little Platte 
presents a somewhat singular structure. At first glance it is a simple syn- 
clinal fold. On the east side are the beds of the Lower Quartzite and 
White Limestone dipping steeply westward, while on the west they rise 
with the slope of the next ridge, which from South Peak southward forms 
the main crest of the range. More careful examination, however, shows that 
the series of beds on either side of the syncline do not exactly correspond, 
and that the change from eastern to western dip is abrupt and not gradual, 
as it should be in a normal syncline. The bottom of the valley, where 
outcrops are visible, shows the Blue Limestone dipping eastward, and above 
it a thin bed of White Porphyry, succeeded higher up by sandstones and 
black shales of the Weber Grits formation. Through the latter, near the head 
of the Little Platte, and just at the boundary of the map, a branch from the 
northeast has cut a deep, picturesque gorge. Climbing the eastern slopes 
of the gorge to the main ridge, across easterly dipping Weber Shales, one 
comes suddenly, at the foot of the steeper slope, upon beds of White Lime- 
stone dipping steeply to the westward. It is evident, therefore, that the 
movement of the Weston fault has been continued somewhat beyond the 
boundary of the map, though it dies out before the Little Platte takes its 
bend to the eastward, just south of this boundary. 

On the summit of Weston's pass the structure can be more clearly seen, 
though it is complicated here by a sudden curve in the beds which form 
the western member of the fold, giving them for a short distance a strike 
nearly east and west, instead of northwest and southeast. This pass, which 
has an elevation of only 11,930 feet, was formerly the main approach to 
Leadville from the east. Its summit is a low saddle, on the east of which 


the steep granite wall of Western's Peak rises over 1 ,500 feet in a distance 
of about half a mile. At the very summit of the pass is a thin bed of 
White Porphyry, overlying considerable outcrops of Blue Limestone, very 
much metamorphosed and iron-stained, and dipping from 35 to 45 to the 
north and east. West of this the underlying White Limestone and Lower 
Quartzite sweep up, at a gradually shallowing angle, almost to the very 
summit of South Peak. On the eastern side of the pass black shales and 
quartzitic sandstones of the Weber Grits can be traced for several hundred 
feet up the face of the slope. These are suddenly cut off by a bed of 
white quartzite, standing at an angle of 70 to the westward, and suc- 
ceeded on the east by granite and gneiss. Between the two is the line of 
the Weston fault. Following this line southward around the angle of the 
upper spur to the basin at the foot of Weston's Peak, the quartzite becomes 
steeper and finally bends over with an angle of f>0 to the westward. The 
actual fault line cannot be traced, inasmuch as it is covered by the talus 
slope. The thin bed of fine-grained brown conglomerate which forms the 
base of the Lower Quartzite, in contact with the Archean, is, however, not 
to be mistaken. In the Archean itself there seems to be a tendency to a 
bedded structure parallel with this lower bed of the Cambrian, and, more- 
over, a sort of actual passage from sedimentary into crystalline rocks, as 
shown by an increasing development of well-defined crystalline feldspars. 
These transition beds pass into a peculiar granite of yellowish-red color. 
It belongs to the coarsely crystalline type, and apparently owes its color to 
the hydration of the oxide of iron, which gives the flesh-colored tint to the 
orthoclase of the normal granite of the region. 

South Peak ridge. From Weston's pass southward the South Peak ridge, 
which follows approximately the direction of the major strike of the forma- 
tions, viz, S. 20 to 30 E., constitutes the main crest of the range. The 
summit of this ridge and its eastern slopes are covered with a thin shell of 
Lower Quartzite beds, whose dip, quite gentle on top of the ridge, steepens 
to 45 on the eastern spurs. Archean exposures cover the whole western 
slope of the range south of Weston's pass and are disclosed in the deeper 
canon cuts on the east side by eros'on of the overlying quartzites. 


From Western's pass to the north base of Buffalo Peaks, a distance of 
about 10 miles, the upper valleys of the Little Platte and of Rough-and- 
Tumbling Creek form a continuous line of depression parallel to this ridge. 
These two streams bend to the eastward and flow together at the south- 
ern end of the Weston's Peak ridge, where the anticlinal fold dies out as 
the ridge disappears under the plain. It is here that the geological struct- 
ure of the range changes from a double anticlinal to a single monoclinal 
ridge, a change which is shown in the varying strikes and dips of the low 
hills at the junction of these streams. Along upper Rough-and-Tumbling 
Creek the Paleozoic beds all dip eastward in apparent conformity, though 
with some variation of angle, and continue' their regular southeast strike, 
not only close up to the base of the Buffalo Peaks mass, but apparently be- 
yond it, without any sensible change of direction. It would appear, there- 
fore, that the flows of andesitic lava, of which these peaks consist, have 
been poured out through the upturned strata and spread out across their 
edges, covering thus a geological horizon extending from the Archean up 
to the Upper Carboniferous, or possibly the Trias, in marked contrast to 
the manner in which the intrusive sheets of the earlier eruptives have been 
formed. 1 

Western slopes. From Weston's gulch southward bey ond the limits of the 
area mapped, the western slopes of the range are composed of Archean rocks, 
among which granite is very prominent. There are doubtless many eruptive 
dikes cutting through them in this area besides those of White Porphyry 
at the mouth of Granite Creek, represented on the map, but time did not 
admit of a sufficiently detailed examination to determine their outlines and 

Weston's gulch, below the junction of its two heads or forks, which run 
with the strike of the formations northeast and southwest, is a straight nar- 
row gorge cut out of Archean granite and gneiss.. Its form suggests partial 
glacier carving, but later erosion has removed all traces of moraine mate- 
rial except a few erratics. Below this narrow gorge it passes into an open 
country, occupied by partially eroded terraces of the Quaternary Lake 

'A more detailed description of the Buffalo Peak region will be found in Bulletin No. 1, United 
States Geological Survey, Washington, 1883. 


beds, which will be described later. The Archean area, with its covering' 
of Lake beds on the lower spurs, extends north as far as Empire gulch, 
but beyond that line it no longer outcrops, except where brought to the 
surface by faulting and erosion in the deep amphitheaters near the crest of 
the range. 

Weston fault. From Weston's pass northwestward the Weston fault 
follows the foot of the steep western slope of the main crest of the range, 
approximately parallel to and a little east of the valley bottoms of the two 
forks of Weston's Creek. East of it are Archean exposures, capped, either 
on the crest of the range or on its eastern slopes, by easterly dipping Pale- 
ozoic beds. To the west is a fringe of successive outcrops of the same 
beds, also dipping regularly eastward, whose varying outlines, as shown on 
the map, are entirely due to the relative depth of erosion of the various 
gulches Were the structural conditions studied on a single transverse line 
in this area they would be naturally supposed to be those of a simple 
monoclinal fault; but the unmistakable evidence of the synclinal fold, as 
already described on Weston's pass, and the conditions found on Empire 
Hill, which will be described below, show that before erosion had removed 
it there must have been a fold somewhat as indicated by the dotted lines in 
Section G, Atlas Sheet IX. From the pass down the south branch of 
Weston's Creek nearly to the forks, the Lower Quartzite extends up the west 
slopes of the valley, while the Blue Limestone forms a decided shoulder on 
the eastern slopes ; the White Porphyry body, which is only about twenty 
feet thick at the pass, thickens to the northward and by its white color 
forms a prominent feature in the landscape. 

Just above the forks the north branch of Weston's Creek runs in a 
narrow ravine, in which the dip of the beds is somewhat steeper than in the 
south branch, which may be explained by its proximity to the fault line. 
On the northwest side of. this ravine the Paleozoic beds sweep up on a 
broad flat shoulder, which forms the southern continuation of I^mpire Hill, 
gradually assuming a shallower dip as they extend farther westward. 

Empire Hiii. This name is given to the upper part of the spur between 
Weston's and Empire gulches and the broad shoulder or secondary ridg-e 
lying between the branches of these gulches and the head of Union gulch. 


Along the steeper Avestern face of this shoulder the Cambrian and Silurian 
outcrops rest on the Archean, and the top of the shoulder is at the contact 
of the Blue Limestone and overlying White Porphyry, which has been 
quite extensively prospected, without, however, disclosing any considerable 
ore bodies. 

At the head of the north branch of Weston's gulch the ridge which sep- 
arates it from Empire gulch presents a steep slope to the southward, which 
affords a good section of a series of limestones, shales, and sandstones in a 
thickness of from 300 to 600 feet, belonging to the Upper Coal Measures, 
from which the following fossils were obtained: 

Arckceoccidaris, sp. undet. 
Polypora, sp. undet. 
Synocladia, sp. mulct. 
Rhombopora lepidodendroides. 
Palceschara, sp. undet. 
Streptorynchus crassus. 
Chonetcs granulifera. 
Productus costatus. 

Macrocheilus ventricosus. 
Phillipsia, sp. uudet. 
Productus Nebrascenais. 
Chonetes glabra. 
Spirifera Rocky montana. 
Athyris subtilita. 
Productus Prattenanus. 
Nucula, sp. undet. 
Astartella, sp. undet. 

Section F, Atlas Sheet IX, which passes through the ridge, shows the 
structural conditions which prevail here. Where these beds join the fault 
they stand quite vertical and give evidence of having been subjected to great 
pressure, as shown in the specimen represented in Fig. 2, Plate V (page GO) ; 
but at a little distancefrom the fault it can be seen that near the top of the ridge 
they gradually bend over to the westward, until a few hundred yards west 
they assume the normal dip of 20 to the eastward. Below these, both on 
the ridge and in geological succession, are the sandstones of the Weber Grits, 
which form the mass of a low rounded hill. Along the western face of this 
hill, and immediately above the White Porphyry, is a considerable thick- 
ness of compact black argillaceous shale, impregnated with pyrites. This 
black shale has been opened in several places by prospect holes, and from 
it were obtained numerous casts of fossils, in which the calcareous matter 
of the original shell has been entirely replaced by very minute crystals 
of iron pyrites, so minute that the form of the shell is still distinctly 
visible in those which are newly opened, though they rapidly decompose on 



exposure to the air. The contrast of the glittering yellow of the pyrite 
with its dull-black background of shale is extremely beautiful. This bed 
of black shale represents the base of the Weber Shale formation. From 
it were obtained the following forms : 

Discina Meeki. 
Orthis carbonaria. 

Streptorhynchus crassug. 
Aviculopecten rectilaterarius. 

Chonetes granulifera. 

Below the black shales is the main sheet of White Porphyry in consid- 
erable thickness, succeeded by the Blue Limestone which forms the eastern 
edge of the spur or shoulder, while the White Limestone and underlying 
quartzite can be traced along the steep slopes below. The series is here, 
therefore, complete from the Lower Quartzite up to the Upper Coal Meas- 
ure ; and, even had the fossils obtained in the latter not been found, the ex- 
istence of such considerable thicknesses of limestone above the Weber 
Grits would have been enough to determine their horizon. The gradual 
passage observed from the shallow dip of 20 to the vertical dip adjoining 
the fault is proved by actual observation and furnishes an analogy for the 
vertical dips already observed at the London fault. The fault line itself is 
exposed in a tunnel and is exceptionally distinct on the ridge, its direction 
being here N. 25 to 30 W. ; the adjoining rock on the east is a coarse- 
grained granite and on the west shales and grits. Where opened, the fault 
shows slickensides and a considerable development of clay selvage, with a 
fine breccia of very dark color, the result of friction. In the granite ad- 
joining the fault there is visible decomposition for some ten or fifteen feet, 
consisting in a partial kaolinization of the feldspars and a hydration of what- 
ever oxides of iron it contains, which is evidently due to the action of 
waters which have followed the plane of the fault. 

In the basin at the head of the north fork of Weston's gulch, only 
a few feet east of the line of the fault and apparently parallel with it, is a 
vein of quartz in granite, some six or eight feet in thickness, which can be 
traced up the wall of the ridge. In the first saddle of the ridge above Empire 
Hill is a dike of White Porphyry about twenty feet thick, in the vicinity of 
which the granite is decomposed in a manner similar to that near the Weston 
fault. This saddle is on a line with the fault which runs between Sheridan 


and West Sheridan, and although at this point, owing to the fact that gran- 
ite is on either side and the surface is largely disintegrated, the fault could 
not be visibly distinguished, it is supposed that the Sheridan fault crosses 
this saddle to connect with the Weston fault. The White Porphyry dike 
would thus at first glance seem to be due to an eruption which had taken 
place along the pjane of an already existing fault ; but the evidence obtained 
elsewhere all goes to show that the time of eruption of the White Porphyry 
was entirely antecedent to the action of faulting; and it is therefore more 
probable that the White Porphyry dike had followed a line of weakness or 
possible fracture, which in the subsequent dynamic movements would have 
been more susceptible to faulting than other portions of the formation. 

Between the head of Union gulch and Empire gulch, below the steeper 
slope of Empire Hill, is a triangular area in which are relics of the lower 
Paleozoic series, with included porphyries, which have been folded and 
faulted in an extremely intricate manner. A simple expression of their 
structure is shown in Section E, in which it is seen that at the foot of the 
steep slope of Empire Hill a second fault has cut off a portion of a synclinal 
basin. The upper member in the trough of the syncline is the White Por- 
phyry, immediately overlying the Blue Limestone. A shaft has penetrated 
this porphyry into the Blue Limestone below. On the east of the syncline 
the beds dip 25 to the westward, while on the west side they dip at an 
average of 10 to the eastward. ' The southern extremity of the fault is seen 
near the forks at the head of the north branch of Union gulch, where a little 
patch of Lower Quartzite rests against the fault, with granite on either side. 

Here occurs a singular eruption, apparently in the form of an inter- 
rupted dike, of a rock whose lithological characters ally it to the Tertiary 
eruptives. It has been colored on the map as a rhyolite, though it might 
more strictly be classed as a quartziferous trachyte. It is a rather fine- 
grained grayish rock, of thoroughly trachytic texture, whose most prominent 
elements are small glistening hexagonal leaves of biotite ; a few rounded 
grains of quartz are also visible, and the rest of the rock is made up of small, 
rather glassy grains of feldspar. Between the crystalline elements is an ill- 
defined groundmass of gray color. The rock has included fragments of 
quartzite. Parts of the groundmass are truly microfelsitic, and in some 


places undoubted glass substance is present. The rock also contains frag- 
ments of another eruptive rock, in some respects resembling the Gray Por- 
phyry, and in whose cryptocrystalline groundmass are numerous aggre- 
gates of tridymite. 

From the head of Union gulch northward, on the west of the syncline, 
the outcrops of Lower Quartzite and White Limestone grow wider, owing 
to shallowing dip, till they are cut off by the valley of Empire gulch, 
and are succeeded on the west by the underlying granite. On either side 
of the syncline the Blue Limestone forms prominent outcrops or ridges. 
On the north end of the syncliue, toward the ravine which runs down to 
a little lake adjoining the meadow of Empire gulch, is a small body of 
Gray Porphyry, apparently occurring between the Blue and the White 
Limestone. Following the line of the fault northward from the head of 
Union gulch, the Lower Quartzite, White Limestone, and Blue Limestone 
are found successively in contact with the granite; and tinally the White 
Porphyry almost touches it. Farther north the series is reversed, until in 
the bed of the ravine at the foot of the north end of Empire Hill granite 
is exposed on either side of the fault. There is here an anticlinal fold 
whose axis corresponds with the major strike and from whose crest the sed- 
imentary series have been removed down to the Lower Quartzite. Con- 
tinued north, the line of the axis of this anticline nearly corresponds with 
the Mike fault, which is first seen on the north wall of Empire gulch and 
which will be described in detail in the chapter devoted to the vicinity of 
Leadville. 1 

Union fault, which thus far has followed the foot of the steep slope of 
Empire Hill, now cuts across the northwest spur of this hill, and beyond 
Empire gulch, after crossing Long and Deny Hill, joins Weston fault. The 
displacement of this fault, like that of most of the faults of the region, is an 
upthrow to the east. Consequently in ascending the steep northwest spur 
of Empire Hill from the meadows in Empire gulch or from the anticline 
above mentioned, one crosses a double series of easterly-dipping lower 

'By an error of the engraver, overlooked iu proof-reading, the line of Mike fault has been carried 
across Empire gulch to a connection with Union fault, following what was intended to be simply the 
dividing line between the Cambrian and Siluria:; formations. 


Paleozoic beds. At the foot of the steep slope, between the Lower Quartz- 
ite and the White Limestone, is a small body of eruptive rock whose out- 
crops are so obscure that its structural relations could not be accurately 
determined. It is a fine-grained, nearly white rock, with minute specks of 
biotite and small white feldspars macroscopically visible as porphyritic 
constituents. Microscopical and chemical examinations show it to be an 
orthoclastic rock, containing 68 per cent, of silica. Glass inclusions occur 
in both quartz and feldspar, but no fluid inclusions. It has been classed as 
a rhyolite and is chiefly interesting on account of its isolated occurrence 
and want of resemblance to any other rocks found in the region. 

Empire gulch. Empire gulch is one of the glacier-carved valleys of the 
western slope of the range. At its head is a grand amphitheater cut out of 
granite and gneiss, with a rim of sedimentary strata and intrusive porphyry 
sheets crowning its wall. Two faults theoretically cross its upper portion 
the Sheridan fault and the Mosquito fault which, however, are not visi- 
ble in its Archean bed, as there is no distinction in the character of the 
rock on either side to mark their position. At the Weston fault, however, 
the Lower Quartzite occurs in the bed of the gulch, with an eastern dip, 
and its outcrops sweep up the wall on either side ; these outcrops are par- 
tially masked by two very well defined lateral moraines which border the 
immediate bottom of the valley. 

On the south side of the gulch, in the basin inclosed by the north arm of 
Empire Hill, is a shallow glacier lake, dammed up by one of these moraines. 
In this basin prospect holes prove the existence of black shales and overlying 
Weber Grits above the lake, while below it is the Blue Limestone, succeeded 
by the White Limestone and Lower Quartzite, the line of the Union fault 
being marked by the sudden appearance of White Porphyry, which adjoins 
either of these two formations. Above the White Porphyry, on the steep 
slope at the north point of Empire Hill, immediately west of the fault, is a 
little remnant of Weber Shale. 

The moraine ridges terminate about a mile below (his north point of 
Empire Hill. Here the valley of Empire gulch opens out into a broad al- 
luvial meadow, below which it is cut mainly out of Quaternary Lake beds, 


and consequently loses the distinctive form due to glacial erosion. The 
succession of beds crossed in descending the ridge from the north point of 
Empire Hill to this meadow is sufficiently indicated on the map. 

On the north side of the gulch the structure is even more complicated 
than on the south, and the rock surface is more obscured by morainal and 
other detrital material. Were it not for the numerous prospect holes this 
structure could hardly have been unraveled. It is shown in much more 
detail on the large map of Leadville and vicinity, and its description is re- 
served for the chapter which treats of that region. 

Leaving aside then, for the moment, the region included within the 
limits of this map, the crest of the range and that portion of its western 
slope not included therein will next be described. 

Main crest north of Ptarmigan Peak. At Ptarmigan Peak and for some dis- 
tance north the entire ridge is composed of Archean, in which granite and 
coarse porphyritic gneiss are the main components; thence north to Horse- 
shoe Mountain successive shells of Lower Quartzite, White Limestone, 
Parting Quartzite, and Blue Limestone form the crest. Round the head of 
Empire gulch their outcrops form a semicircular rim, sweeping round the 
western point of Mount Sheridan, while the crest of the ridge is covered by 
the main body of White Porphyry. Under Peerless Mountain a second 
body of White Porphyry comes in between the Blue and White Limestones, 
and extends as far north as the base of Dyer Mountain, where it seems to 
pass down to successively lower horizons, until in Dyer amphitheater it is 
found quite at the base of the lower Paleozoic series. Remnants of this 
second body of White Porphyry form the cap-rock on the western spur of 
Mount Sheridan, known as West Sheridan, whose mass, by the slight move- 
ment of displacement of Sheridan fault which runs through the saddle sep- 
arating these two peaks, has been let down relatively to the mass of Mount 
Sheridan itself; in other words, its upthrow is to the eastward. This rather 
singular fault passes partly across the head of Iowa Amphitheater, where 
it is joined by a fault at right angles to it, or running nearly east and west; 
as the result of their movement, a little segment of beds of the Lower 
Quartzite, White Limestone, and overlying White Porphyry is left in the 


bed of the gulch at the entrance to the north branch of this amphitheater, 
their northern and eastern continuations being found near the top of the 
adjoining cliffs. 

Lake beds. From a little south of the mouth of Weston's gulch north to 
the valley of the East Arkansas, the gently-sloping, flat-topped, lower spurs 
of the range are formed of the Lake bed deposits already described. Actual 
outcrops of these are only found in the southern portion of this region, as 
in the neighborhood of Leadville they are covered by rearranged moraine 
material, which has received the local name of "Wash." The best oppor- 
tunities for observing these within the area of the map are on a narrow 
ridge south of Little Union gulch and along the south wall of Lower Empire 
gulch. At the former locality is exposed a thickness of 300 feet of outcrop- 
ping beds sloping regularly 3 to the westward, which is also the slope of 
the adjoining mesa-like ridges. They consist of gravel and coarse sand, al- 
ternating with beds containing large subangular or partially rounded frag- 
ments of the various rocks which make up the higher portions of the range. 
On the top of the ridge facing lower Empire gulch they have been opened 
by prospect holes, and show a conglomerate with lime cement overlying a 
bed of granite sand, with one iron-stained streak between. Here the dip is 
still 3 to the westward, but in the bed of Empire gulch, where the stream is 
deflected from its course by a knob of Archean granite projecting about 150 
feet above the valley, they are found to have a dip of 15 to the northeast, 
showing that there has been some local movement since they were deposited. 
There are several outlying patches of these beds left high up on the spurs 
in the region shown on the Leadville map. Since the presence of the 
beds within this area could only be proved by underground workings, the 
outlines there given are necessarily somewhat hypothetical, and may be 
subject to change when they shall have been pierced by shafts at other 
localities. The highest points at which their existence has been proved 
in this area are on the western slopes of Long and Deny and Printer Boy 
Hills, respectively, where they extend up to the 11,000-foot curve. This 
is just 1,000 feet above the outcrops between Little Union and Empire 
gulches, and higher than the dip of 3, already u very considerable in- 
clination for an average angle of deposition over a large area, would carry 


them; this, taken in connection with the observed angle of 15, leads to 
the inference that the mountain mass has been elevated to some degree 
above the Arkansas Valley since the beds were deposited. 


The northwestern division comprises the area west of the Mosquito 
fault and north of the area of the Leadville map. It is a region which is 
comparatively unbroken by faults, and from its lower elevation one in which 
the structure lines are more difficult to read, owing to want of continuity in 
the outcrops. Between this and the area last described lies the complicated 
region represented on the Leadville map, which will be treated in the fol- 
lowing chapter. Its broad general features, so far as are necessary for the 
comprehension of what follows, may be given in a few words. The sedi- 
mentary beds, within which was an enormous development of eruptive 
rocks, largely in the form of intrusive sheets, have by the force of contrac- 
tion been com ressed into a series of anticlinal and synclinal folds, and 
broken by tiansverse fractures or faults, only two of which extend out to 
any considerable distance beyond this area, viz, the Weston fault on the 
south and the Mosquito fault on the north, which are practically part of one 
great displacement. As shown on the western ends of Sections D and E, 
by these faults the area is broken into blocks, which have been successively 
lifted one above the other toward the crest of the range. These faults have 
in general some definite relation to the axis of the folds, and as they pas& 
northward merge into them. Thus out of the six faults represented on Sec- 
tion E two have already disappeared before reaching the line of Section D, 
and on the line of Section C, which passes through Prospect Mountain 
along the northern edge of the Leadville map, the structure has simplified 
itself into two broad anticlines, with an included syncline, and there is no- 
visible fault west of the Mosquito fault. 

Prospect Mountain. The surface of the massive of Prospect Mountain, 
which lies between Big Evans and Bird's Eye gulches and extends from the 
Mosquito fault to the east fork of the Arkansas, is covered to a considera- 
ble depth by broken masses of porphyry and of sandstones and schists of 
the Weber formation, so that but few outcrops are found. Fortunately 


there are many prospect holes, showing the character of the rock beneath 
the covering of de'bris, by means of which the general outlines of its struct- 
ure can be determined. These are shown in Section C, Atlas Sheet VITI, 
and in Section K, Atlas Sheet X, the former of which follows the crest of 
the ridge in an east and west direction, while the latter crosses it in a north- 
west direction. From these it is seen that from the summit of Prospect 
Mountain eastward to the Mosquito fault the surface is occupied by beds 
of the Weber formation, with a general easterly dip. They are crossed by 
a few dike-like masses of eruptive rocks, the most prominent of which are 
two dikes on the crest of the ridge : the one a coarse-grained quartz-por- 
phyry, with large crystals of orthoclase, which resembles a Gray Porphyry ; 
the other a fine-grained micaceous rock resembling a diorite, but yet con- 
taining a large proportion of orthoclastic feldspar. The latter rock also 
occurs on the north slope of the massive near the head of Indiana gulch, 
and in an important body at the mouth of Bird's Eye gulch where it de- 
bouches into the East Arkansas Valley. West of the summit of Prospect 
Mountain, however, the slopes toward the adjoining valleys are very steep 
and cut through the Weber beds, disclosing a somewhat complicated anti- 
cline, or rather the intersection of two systems of anticlines, and the devel- 
opment of a large body of porphyry found only in this mountain and on 
Mount Zion, which is separated from it by the deep cut of the East Ar- 
kansas Valley. This porphyry, which has already been described as a more 
crystalline variety of the White Porphyry and which is designated by the 
color of that rock on the map, is called, from the locality of its principal 
development, Mount Zion Porphyry. 

Mount Zion Porphyry. This porphyry is exposed in great thickness under 
the Weber Grits on Mount Zion and on the northeast slope of Prospect 
Mountain; it is also denuded at the head of the north fork of Little Evans 
by the deep erosion of the gulch. It apparently replaces in part the main 
sheet of Gray Porphyry, which directly underlies the Weber Grits to the 
north and south of it and thins out as the former grows thicker. For 
this reason it has been indicated on the sections as a rapidly thickening 
sheet, though it is not at all improbable that it may be a laccolitic body, 
like those of White Ridge and Gemini Peaks, and have its vent, or channel 


through which it came up, somewhere under Prospect Mountain. It varies 
much in external appearance : in its most unaltered form, as obtained at a 
depth of 200 feet in the Hattie bore-hole, it resembles a fine-grained granite 
or granite-porphyry, while in the extreme of alteration, as found in some of 
the shafts on the southwest slope of Prospect Mountain, it is hardly to be 
distinguished from decomposed White Porphyry. It differs microscopically 
from the fine-grained granites by the absence of microcline and by the 
presence of prismatic microlites of plagioclase with rounded ends, which 
are particularly abundant in the quartz and orthoclase. 

The structure of the southern slopes of Prospect Mountain, which is 
somewhat complicated, is described in detail in Chapter V, and the relations 
of the White and Mount Zion Porphyries are shown on the map of Leadville 
and vicinity, where they have distinct colors. The outcrops of the thin 
sheet of the former and of the underlying Blue Limestone, which occur 
along the East Arkansas Valley at the foot of Prospect Mountain, are only 
proved by prospect holes, the actual rock surface being buried under debris 

Mount Zion. The mountain mass of Mount Zion and its southwestern 
shoulder, known as Little Zion, presents a somewhat similar structure to 
Prospect Mountain, of which it originally formed a part, and shows better 
outcrops by which to trace its geological structure. Towards the valley of 
the East Arkansas, on the southeast face of Little Zion, are fine cliff sec- 
tions showing an arch of Archean, over which the Paleozoic beds and 
included sheets of porphyry are folded, with a steep dip to the northeast 
and a more gentle one to the southwest. Along the south face of Little Zion 
the Blue Limestone outcrops can be distinctly traced, gradually descending 
the hill with a southeast dip until, opposite the brewery in the Arkansas 
Valley, they come down to the level of the flood plain and furnish raw 
material to several lime-kilns. At the western extremity of the Little Zion 
Ridge, beyond the limits of the map and opposite the junction of the East 
fork with the Tennessee fork of the Arkansas, is a little hill of granite, 
which is remarkable as being the only place where direct evidence is 
afforded of any considerable inequalities in the Paleozoic ocean bottom. 


In every other case where the junction of the Paleozoic outcrops with 
the Archean has been observed, practically the same bed of quartzite has 
been .found at the contact. In this case, however, on the saddle east of 
this little hill, the White Limestone is found to abut against the granite, 
while the Lower Quartzite sweeps around its northwest and southwest 
slopes, showing that this point projected as a submerged island above the 
level of the Cambrian beds at the time of their deposition. In the bed of 
the stream opposite this saddle the Lower Quartzite beds are exposed in a 
little canon gorge, with a strike of N. 30 E., and dipping 20 to the 
southeast under the Lake beds which cover the spurs up to the steeper 
slope of the hills near Leadville. 

The valley above has alluvial meadows, with flood-plain benches on 
either side. In these benches on the northwest side is the Dugan quarry, 
whence limestone was formerly taken as a flux for the Leadville smelters. 
Higher up the valley, where the beds bend up over the arch of Archean, a 
careful measurement was made of the cliff-exposures at two points, which, 
in descending order, are as follows : 

1. Cliffs back of toll-gate. 


Wliite Porphyry . . - ? 

Quartzite and shale 25 

Lower Carboniferous . ^ Blue Limestone, with chert at top and bottom and 

with breccia at the base 125 


Sandstone, eroded, with limestone breccia in eroded 

hollows 15 

Space covered 15 

Bluish limestone 8 

Silurian ( Wliite Limestone, bluish at base C6 

Sandy limestone 28 

White calcareous quartzite 22 

Reddish sandy limestones 27 

Limestone, gray at top, white at bottom 40 


De"bris slopes to valley bottom. 


2. Section across arch of Archean. 

Cambrian . 

1 Debris slopes above cliff. 

Saccharoidal quartzite, white and thin-bedded. CO 

Saccharoidal quartzite, like above, but stained and 

discolored 60 

Coarse quartzite, with fragments of feldspar 1 


Aichean Eed granite ; upper beds very much decomposed ; 

red feldspars turned yellow by hydration of iron 
oxide; 250 feet to base of cliffs. 

The White Limestone seems relatively thicker and the Blue Limestone 
thinner than usual. The evidence of erosion on the sandstone underlying 
the latter, which consists in hollows and ridges two or three feet in depth 
or height filled by a limestone breccia, is important as indicating a land 
surface at the close of the Silurian. Unfortunately this was the only point 
at which it was detected, so that it cannot be said with certainty that the 
land elevation at that time was very widespread, although the apparent 
absence of Devonian beds is indirect evidence that it was, as is also the 
great variation observed in the thickness of the upper member of the Silu- 
rian, the Parting Quartzite. 

The White Porphyry above the Blue Limestone has a maximum thick- 
ness of about fifty feet on the west point of Little Zion and rapidly wedges 
out to the north and east It has the usual appearance of the normal rock, 
but the fresh-looking hexagonal crystals of dark mica are rather more 
abundant than usual, for which reason they were separated and analyzed, 
and found to be muscovite instead of biotite, with which determination their 
optical properties agree. (See Appendix B, Table I, Anal. II.) Above 
this is the Gray Porphyry, which readily disintegrates and crumbles into 
coarse sand, and therefore can be traced along the west slope of Mount 
Zion by the gentle slope which it forms at the foot of the steeper slope of 
Mount Zion Porphyry above it. It has here a thickness of about 100 to 
150 feet, which increases to the northward; it evidently connects with the 
immense sheets of Eagle River Porphyry at the northern limits of the map. 
The Mount Zion Porphyry has a thickness of about 800 feet on the crest 
of the ridge, but rapidly wedges out to the north. In its upper part, near 


the summit of Mount Zion, is an included sheet, about one hundred feet 
thick, of sandstones and shales of the Weber formation, which can be traced 
down the south slopes to the East Arkansas Valley. 

Tennessee Park. From Little Zion northward the Lower Quartzite beds 
form a flat shoulder along the lower slopes of Mount Zion facing Tennessee 
Park for a distance of several miles. Below this shoulder the steeper slopes, 
scored by shallow ravines, are in the granite of the Archean, while above 
are successively White Limestone, Blue Limestone, and Gray Porphyry, 
with Weber Grits capping the whole and covering all the hills to the east 
and north. Between No Name and Tennessee gulches there is a discrep- 
ancy in the outcrops of the lower beds, which can only be explained by 
a fault, approximately as shown on the map, by which their northern con- 
tinuation is thrown more to the westward. All these western slopes are 
thickly covered with timber, and it is not always possible to determine 
accurately the outlines of the formations. Tennessee gulch heads on the 
western slopes of Buckeye Peak and, flowing first westward past Coop- 
er's Hill, takes a bend to the southward, afterwards bending again west- 
ward into the open valley of Tennessee Park, beyond the limits of the 
map, where it joins the main branch of the Arkansas, which descends from 
the slopes of Homestake Peak. Between the south bend of Tennessee gulch 
and the main Tennessee Valley, just west of the map, is a low ridge of 
granite, gradually covered, as one goes north, by nearly horizontal beds of 
Lower Quartzite. These beds can be traced across Tennessee pass west- 
ward to the northern flanks of the Sawatch Range, where they cover the 
spurs extending northwards to the valley of Eagle River. 

Along the western borders of the map northwards from Tennessee 
gulch a fringe of outcrops of lower Paleozoic beds follow the foot of Cooper's 
Hill and cross the upper valley of Piney Creek, which flows into Eagle 
River through Tennessee pass. The body of Gray or Eagle River Por- 
phyry overlying the Blue Limestone becomes very much thicker in this 
region, and on the slopes of Buckeye Hill rises in horizon, leaving a portion 
of the Weber Grits formation, consisting of shaly beds, beneath it. On El 
Capitan Creek there is also a portion of the Weber Grits included in the 
body of porphyry. On Taylor Hill, north of the head of Piney Creek and 


just beyond the extreme northwestern corner of the map, is the El Capitan 
mine, which is of interest as being the only considerable ore deposit thus 
far developed in this region at the Blue Limestone contact In the Weber 
Grits, which form the surface rocks from Piney Creek eastward across Chi- 
cago Ridge to Chalk Mountain, as shown in Section A, are numerous bodies 
of porphyry, which doubtless originate in an immense laccolitic body which 
occurs just north of the limits of the map, near the head of Eagle River, and 
on a line with Chicago Ridge. 

East Arkansas valley. The flood-plain deposit, which forms benches on 
either side of the alluvial bottom of the east fork of the Arkansas, extends 
up a little distance beyond Rowland post office, above the lower bend. 
Under this no rock outcrops are visible. The south wall of the valley, 
formed by the slopes of Prospect Mountain, is mostly covered by debris, 
but the north wall on the Mount Zion side has cliff-faces and abundant out- 
crops. Above the arch of Archean, already described, the successive beds 
of the lower Paleozoic series, the Gray and Mount Zion Porphyries, and 
the thin bed of Weber Grits included in the latter descend into the valley 
at a steep angle. The dip of the beds rapidly flattens out, however, as one 
ascends the valley, and near the mouth of Buckeye gulch the Weber Grits 
have become nearly horizontal. 

Between Buckeye gulch and the bend of the valley below Howland an 
important body of Lincoln Porphyry, with characteristic large orthoclase 
feldspars, comes in, which can be traced up the valley wall for a distance 
of two miles, apparently conformable with the bounding beds of Weber 
Grits. A similar body exists on the east side of the valley, extending from 
the mouth of Bird's Eye gulch up to a terminal moraine ridge half way 
between Howland and Chalk ranch. It would seem that these two out- 
crops are parts of the same great sheet of porphyry, though their connection 
across the valley is not very distinct. The prevailing dip of the inclosing 
sandstones is generally to the southeast. On the ridge between Arkansas 
Valley and Buckeye gulch this dip is quite pronounced and in places as 
steep as 45. Towards its north end the eastern body forms a prominent 
hill, called the Dome, with a steep face toward the valley, which shows a 
tendency to columnar structure. The porphyry body is here much thicker 


than at any other point, and it is not improbable that this is the laccolite 
from which the rest of the body has spread out. The rock of this porphyry 
mass (72) is a fresh-looking, light-gray rock, containing large pinkish 
crystals of orthoclase, abundant quartz (showing generally a crystalline 
form), and small hexagonal leaves of biotite. The groundmass is gray and 
subordinate to the crystalline constituents in quantity. Under the micro- 
scope it is seen that nearly half of the smaller feldspars are triclinic and 
much altered, while the larger ones are comparatively fresh. This por- 
phyry is as nearly the equivalent of the Lincoln as of the Eagle River 
type, and is one link in the chain of evidence showing that all these allied 
types constitute one large group. (See Appendix A.) 

At the base of the Dome is an outcrop of quartzite dipping to the 
southeast, which rises as one follows the cliff southward. In the little 
ravine next south is a second body of porphyry, separated from the main 
sheet by quartzites and shales through which it penetrates somewhat irreg- 
ularly; it may be an offshoot from the main body, though it differs some- 
what lithologically and is moreover impregnated with secondary pyrite; as 
it is very much decomposed, its character cannot be definitely determined. 
At the mouth of Bird's Eye gulch the porphyry body has risen to a con- 
siderable height on the ridge; while below it, between the mouth of Bird's 
Eye gulch and Indiana gulch, is a body of the finer-grained dioritic-looking 
porphyry already mentioned, which crosses over into the bed of Indiana 
gulch higher up, in the direction of the dike of the same rock on Prospect 
Mountain Ridge. 

The slopes of Mosquito Range between Bird's Eye and English gulches, 
east of the porphyry body, are mostly made up of sandstones and occasional 
beds of black shale of the Weber Grits formation, whose prevailing dip is 
10 to 15 a little to the south of east. On the summit of the ridge between 
the head of Bird's Eye gulch and the Arkansas Valley, however, the beds 
have a shallow dip to the west, giving evidence of a slight synclinal roll, as 
has been indicated in Section B. 

On the west face of the ridge separating the head of Bird's Eye gulch 
from that of English gulch is an outcrop of limestone, which is probably 
one of the beds that occur in the middle of the Weber Grits series. Asso- 


dated with this is a porphyry different from those hitherto described, and 
characterized by small feldspar crystals of a deep purplish-red color. This 
and a decomposed green mineral, which are its only porphyritic compo- 
nents, lie in a light-green groundrnass. Under the microscope the green 
mineral is seen to be altered to chlorite, so that its original condition cannot 
be determined, though it was probably biotite. The coloring matter of the 
feldspar is a reddish substance in small flakes, possibly oxide of iron. 
From the limestone outcrop eastward to the Mosquito fault the ridge is 
made up of coarse white sandstones, having a gentle easterly dip. Beyond 
the fault line the steeper slopes of the range are made up of fine-grained 
granite, which resembles an eruptive granite. In this about half way up 
the slope is an irregular dike of porphyrite. 

At the mouth of English gulch, just north of the Dome, are several 
bodies of porphyry, and the structure of the sedimentary beds is extremely 
irregular, the dip being rather to the northeast or away from the porphyry 
mass, while on the ridge between English and French gulches the beds dip 
to the west and southwest, giving further evidence of the synclinal fold 
shown in Section B. 

In the lower portion of French gulch the south and west dips still 
continue, and several small bodies of limestone are found between beds of 
quartzite or altered sandstone. About a mile up the gulch, at the Mount- 
ain Lion claim, is a body of diorite of blue-gray color and largely impreg- 
nated with pyrites. It has a thoroughly granitic texture and shows macro- 
scopic crystals of feldspar, hornblende-, biotite, and quartz. At the head of 
the gulch easterly dips again come in; but these change again to the west 
before the fault line at the foot of Mount Arkansas is reached, showing a 
second syncline, which may be a continuation of the syncline adjoining 
the fault that is so well developed at the north edge of the map, though 
the general strike of that fold would carry it to the west of this. On the 
divide between the head of French gulch and the Arkansas was observed 
a body of Lincoln Porphyry, opened by a prospect hole to a depth of twenty 
feet or more, which is so thoroughly disintegrated that when cut down by 
a pick it crumbles in the hand. 


Buckeye Peak. On the west of East Arkansas Valley, between it and 
Eagle River, is a broad-topped mountain mass, whose highest point is Buck- 
eye Peak, at the head of the gulch of the same name. To this peak, on the 
Hayden map, no name is given, but a minor point of the ridge to the north 
is called Mount Arkansas a name which -has been given by the miners, 
with more propriety, to the prominent peak west of the Arkansas amphi- 
theater; this transfer of the latter name has therefore been adopted on the 
present map. On the south face of Buckeye Peak, forming the wall of its 
amphitheater in a height of nearly 1,000 feet, is exposed a great mass of 
Eagle River Porphyry, whose prominent vertical cleavage planes and joints 
give the appearance of columnar structure observed on the summit of Mount 
Lincoln. On the debris-covered slopes and grassy ridges its outlines could 
not be traced with perfect accuracy, but it seems probable that it is a lacco- 
lite body, from which the other irregular bodies of the same rock in the 
vicinity may be offshoots. It is somewhat lighter colored than the porphyry 
observed in the Arkansas Valley, and under the microscope shows no glass 
inclusions, but otherwise is identical with that rock. A dike-like offshoot 
from the body extends to the west along the ridge at the head of Tennessee 
gulch. To the south the beds of the Weber Grits formation seem to dip 
away from it for a short distance and then resume their southeasterly dip. 
Above it, on the summit of the peak, these beds lie nearly horizontal. On the 
eastern base of the peak, at the head of the spur which runs down between 
Buckeye gulch and the Arkansas, is a small outcrop of decomposed por- 
phyry, so white that it might be taken for Nevadite or White Porphyry. 
Microscopical examination, however, shows that it is probably a portion of 
the Eagle River Porphyry body. Across' the northern ridge of Buckeye 
Peak runs a dike-like mass of porphyry about 200 feet in thickness, which 
can be traced almost continuously down the bed of Deluionico gulch in a 
steep wall, through which the present stream has cut a steep, narrow bed. 
Its outcrops are obscured by moraine material. This gulch, as well as the 
other main gulches which radiate out from Buckeye Peak, bears evidence 
of having been once filled by a minor glacier, both in the fact that relics of 
moraines can be found along its sides and in that its slope is not such a 



continuous one as would result from the erosion of running water, but is 
comparatively gentle for a considerable distance from the head and then 
descends abruptly into the valley of the Arkansas. 

Along the western slopes of Buckeye Peak, as already mentioned, are 
two principal bodies of quartz-porphyry, apparently interstratified in the 
Weber Grits, which extend northward beyond the limits of the map. East 
of these are several irregular bodies of the same rock, whose outlines are 
given on the map with tolerable accuracy. They are probably connected 
in origin with the large laccolitic mass of Eagle River Porphyry which lies 
just beyond the border of the map to the north. They are in part inter- 
stratified, but little satisfactory evidence was obtained bearing upon their 
underground extension. In the basin between Chalk Mountain and Chicago 
Ridge, whose waters drain into Eagle River, a synclinal fold can be dis- 
tinctly traced, which basins up to the southward. Its outlines are well 
marked by an interbedded sheet of Eagle River Porphyry. The inclination 
of the fold is comparatively shallow on the west, though in some places dips 
of as high as 25 are observed; while on the east the angle is still steeper, 
as shown in Section A, Atlas Sheet VIII. In its trough above the por- 
phyry is a small bed of limestone. 

chalk Mountain. From the head of the straight north and south portion of 
Arkansas Valley projects a singular ridge, like a huge railway embankment, 
prominent by its brilliant white color in the somber surrounding of pine 
trees From the material of which it is composed and which, in the fact 
that it is soft and white, has a certain resemblance to chalk, the person who 
first settled at its base gave to his home the name of Chalk Ranch. At 
this point the Arkansas Valley bends sharply to the eastward and its level 
rises abruptly 100 feet or more; while the direct northern continuation of 
the valley below is formed by a still more elevated valley, the bed of a 
little stream known as Chalk Creek, which a short distance above Chalk 
Ranch falls in a picturesque cascade from the upper valley level into a 
deep narrow basin and flows in a narrow gorge to join the Arkansas just 
bjlow the ranch. The Denver and Rio Grande narrow-gauge railway, in 
order to gain grade enough to overcome this sudden elevation of the valley 
level, climbs gradually along the western wall of the Arkansas Valley, 


reaching the gorge a few feet below the falls, and then curves sharply to 
the east, spanning the chasm with a picturesque bridge, and, emerging 
through a 66-foot cut in the ridge beyond, reaches the south slopes of 
Chalk Mountain completely above the Chalk Ridge. 

Between the valley of Chalk Creek on the west, the Arkansas Valley 
on the south, and the head of Ten-Mile Creek on the east, is a table-topped 
mountain of rudely triangular shape, presenting steep escarpments to the 
south and east. This low mountain mass, which forms part of the con- 
tinental divide, is also formed of a very light-colored eruptive rock, and has 
from this fact received the name Chalk Mountain. Both the rock of Chalk 
Mountain and that of the white ridge below are rhyolite, but of somewhat 
different types. The former is coarse in texture, and, though seen to be 
distinctly porphyritic when closely examined, it seems in some cases almost 
granitic in structure. It is of the variety known as "Nevadite." Upon the 
southern and northwestern edges of the plateau of Chalk Mountain the 
surface is strewn with huge blocks of Nevadite, in which the large sanidine- 
crystals, with their brilliant satiny luster, and the dark smoky quartz 
crystals are especially noticeable. Over the remainder of the surface the 
rock is somewhat finer grained, the quartz crystals being the most prominent 

The rock of Chalk Mountain is different from that of all the neighbor- 
ing bodies, not only in the character of its constituents, but in the time and 
manner of its eruption. It has disturbed the adjacent strata to an extent 
not noticed in connection with any other eruptive of the region, and by 
cutting off bodies of Eagle River and other porphyries its later age is 
proven. The masses of debris upon the steep southern slopes cover its 
contact with the sedimentaries, but upon the east, north, and west numer- 
ous outcrops appear, which illustrate the disturbing influence of the Nevadite- 
mass. The map shows an arm of the Weber Grits projecting up the eastern 
slope to the level of the plateau, and in these elevated beds is a dike of 
older quartz-porphyry, cut off to the west by Nevadite. Fig. 1 of Plate 
XIX shows this relation in detail. The strike of the beds is indicated by 
the lining on the sketch, and the easterly dip measures about 70 at the 
extremity of this arm, declining to 30 at the edge of the cliff. South of 


this arm is another .small area of quartzites, which seem to be entirely 
inclosed by the Nevadite. Farther north, upon the eastern side of the 
body, the strata are much disturbed and show varying strikes and dips, 
and upon the northern slopes, within the Ten-Mile district, the strike of 
the Weber beds is found to be east to west, with a northerly dip of 80 
near the Nevadite, which lessens to 25 at a distance of half a mile. At 
the northwest corner of the mass, just beyond the line of the map, the 
eastern extension of the Eagle River Porphyry sheet shown in the synclinal 
fold is found to be cut through by the Nevadite. Along the western con- 
tact steep westerly dips are found in the sedimentary beds, and several thin 
sheets of Eagle River Porphyry seem to be cut by it, but the relations are 
not clear. A branch of Chalk Creek cuts deeply into the Nevadite and 
testifies to the thickness of the body upon this side. 

Three of the smaller outlying bodies of rhyolite are apparent offshoots 
from the Chalk Mountain mass, but the rock of Chalk Ridge is of another 
type, allied closely to the body shown in the synclinal fold on the north 
line of the map east of Ten-Mile Creek. This rock is fine grained, showing 
only a few quartz grains and minute feldspars, and disintegrates readily 
into a gravel-like mixture. The outcrop of Chalk Ridge is only a few hun- 
dred yards in length, forming a sharp point between the mouth of Chalk Creek 
and the Arkansas. Above it are sedimentary beds, dipping at a shallow angle 
to the north and east and inclosing thin beds of Eagle River Porphyry. 

Opposite Chalk Ridge, on the west bank of the creek, a white rock is 
disclosed in a little tunnel, which at first glance might be mistaken for rhy- 
olite, but which on close examination proves to be simply altered Weber 
sandstone, composed of limpid grains of quartz, white muscovite, and kao- 
linized feldspar. In the gorge of Chalk Creek the first outcrops above the 
Chalk Ridge are thinly-bedded limestones and shales. Higher up, where 
the chasm is spanned by the railway bridge, are sandstones and quartz- 
ites, with intrusive bodies of porphyry, generally interbedded, but also 
crossing the strata. In the railroad cut on the eastern side of this gorge a 
section is exposed, showing one of these intrusive masses crossing trans- 
versely the beds and spreading out above. Figs. 2 and 3, Plate XIX, 
represent sketches taken on the spot at the time when the cut was freshly 






Julius Bien 4 Co.lilh. 



s F Kniniona, GeologiHt-in . r 


made. The porphyries exposed here are quartz-porphyries, not closely 
allied to any particular type; -but on the slope of the hill about one hundred 
yards above the cut is an outcrop of lavender-colored rock, which in its fresh 
fracture shows characteristic features of the Eagle River Porphyry. In the 
railway-cut on the western side of the gorge both sandstones and porphyries 
are thoroughly decomposed, but it can be seen that the latter both spread 
out between and cut across the beds. On the spur which extends from 
Chalk Ranch up to the Buckeye Peak ridge the sedimentary beds dip con- 
formably to the northeast at an angle of 20 to 25. This dip continues 
across the creek and up to the Nevadite at the southwest extremity of Chalk 
Mountain, the disturbance produced by it being slight at this point. A blue- 
gray limestone 6 feet thick is seen in the third railway-cut east of Chalk 
Ranch, where it has an apparent northwesterly dip. On the eastern side 
of the Arkansas Valley, just below and also a little above Chalk Ranch, 
the beds dip 45 to the west and southwest. The evidences thus afforded 
in regard to the structure of the sedimentary formations in this region are 
not very satisfactory, showing simply that the beds are much broken up 
and indicating that the influence of the intrusive Nevadite mass has not 
been felt beyond narrow limits. 

Upper Ten-Mile Valley. The remaining area of sedimentary rocks is em- 
braced between Chalk Mountain on the west, the Arkansas Valley on the 
south, and the Mosquito fault on the east. Through it passes the conti- 
nental divide, separating the waters of the Ten-Mile from those of the 
Arkansas. The low, wooded Fremont's Pass has an elevation above sea- 
level of 11,350 feet, and over it passes the Blue River branch of the Denver 
and Rio Grande Railroad, the steep rise of 500 feet from the Arkansas 
Valley being overcome by means of a long loop, as shown by the map. 
Ten-Mile Creek has its head in a small rugged amphitheater in the Archean, 
just east of the Mosquito fault, whence it flows directly west to the base of 
Chalk Mountain, then turns abruptly northwest, and soon after passing the 
limits of the map bends to the north and then to the northeast. The gentle 
slopes near the creek are wooded, and outcrops are rare until the neighbor- 
hood of the great Mosquito fault is reached. 


The geological structure of this small area is the expression of the 
ending of the great synclinal fold which is the predominant feature in the 
structure of the Ten-Mile district on the north. The beds taking part in 
this fold basin up to the southward, as they pass within the limits of the 
Mosquito map. In the central part of the fold occur strata of the Upper 
Coal Measures, the highest horizon represented west of the Mosquito fault 
within the limits of this map. Forming the dividing line between the Weber 
Grits and the Upper Coal Measures is the Robinson Limestone, the tracing 
out of which gave the key to the structure represented. Above the Rob 
inson Limestone is an intrusive sheet of quartz-porphyry, which has also 
been folded, and thus assists in bringing out the basin-like form of this 
fold. The observed dips upon the western and southern sides of the basin 
vary from 25 to 60, the strike curving as shown upon the map. Upon 
the east the proximity of the great fault has somewhat complicated matters 
and the strata dip very steeply westward. Section A A, Atlas Sheet VIII, 
represents the relations at this point. The intrusive quartz-porphyry men- 
tioned does not resemble the Chalk Mountain Nevadite, and yet it is a 
coarsely porphyritic rock, whose prominent constituents are large sanidine- 
like feldspars and well-formed quartzes, and whose general habit is that of a 
comparatively recent rock. The facts that on the one hand this body ap- 
pears as an intrusive sheet which has been folded and that on the other it 
is cut by a rhyolite of the type occurring in Chalk Ridge have led to its 
classification as a quartz-porphyry. In the center of this fold are several 
minor bodies of rhyolite, of quartz-porphyry, and of a hornblendic porphy- 
rite. These rocks, as well as the fold in which they occur, are much more 
important features in the geology of the Ten-Mile district, and the further 
discussion of their relations will be reserved for the report upon that region. 

Mosquito fault. The line of the Mosquito fault is well defined along the 
base of the steep slope of Bartlett Mountain by the abrupt transition from 
sedimentary to crystalline rocks; but farther south, where it crosses the open- 
ing of the Ten-Mile amphitheater and the valley of the Arkansas, owing to 
the covered nature of the surface, its exact location is difficult to determine. 

At the foot of the steep western slope of Bartlett Mountain, and on the 
very northern edge of the map, a small cliff-face of rock juts out from the 


debris slopes east of the fault line. Its material is a silicious rock, more 
or less iron-stained and of somewhat cherty nature, in some places honey- 
combed and porous like the quartzite knob adjoining the London fault on 
Pennsylvania Hill. Its structure lines, though somewhat indistinct, appear 
to indicate a vertical dip in the stratification, and among the fragments at 
the upper part of the cliff are some of limestone, resembling the base, of the 
White Limestone. All this material is too much metamorphosed to permit 
of an absolute identification of its original character, but it evidently belongs 
neither to the Archean on the east side of the fault, nor to the Upper Coal 
Measure beds on the west. It is fair to assume, therefore, that it represents 
a portion of the Cambrian and, possibly, of the Silurian beds belonging to 
the steep western side of the fold, which once arched over the top of Bartlett 
Mountain (as indicated by the dotted line in Section A), and which, by the 
friction and pressure that accompanied the displacement of the Mosquito 
fault, have been compressed and metamorphosed until they are no longer, 
recognizable. Further evidence in favor of this view is found on Little 


Bartlett Mountain, a continuation of the Bartlett Mountain ridge just beyond 
the limits of the map, upon which a fragment of Cambrian beds, consisting 
of characteristic saccharoidal quartzite, is found capping the summit and 
extending part way down the western slope, with a dip of 45 to the north- 
west. They end in a little cliff a few hundred feet above the fault line, on 
which the contact with the underlying granite is well exposed; a bedding 
parallel to that of the quartzite can be traced for some distance into the 
granite, with an apparent arrangement of the feldspars in layers parallel to 
this bedding. The actual contact consists of the usual fine-grained con- 
glomerate, with small rounded pebbles of limpid quartz. The quartzite at 
the summit of Little Bartlett Mountain alternates with Lincoln Porphyry in 
such manner as to make it probable that the latter is a relic of an inter- 
bedded intrusive sheet, 

Arkansas amphitheater. In the A/kansas amphitheater, remarkable for its 
semicircular form and magnificently steep eastern walls, which rise 1,500 to 
2,500 feet above the stream bed, the debris in its bottom gives evidence of 
a very large development of eruptive dikes, among which hornblende -por- 
phyrites and biotite-porphyrites play an important part. Nearly all the 


larger masses of this rock contain numerous rounded fragments of Archean 
schists, gneiss, and granite. One of the most prominent features is a body 
of porphyrite, near the summit of the eastern wall of the amphitheater, which 
can be traced continuously from below as a dark horizontal line. Near the 
head it sends down a branch across the cliffs to the bottom ; and what is 
probably a continuation of the upper branch was observed, as already 
mentioned, in Buckskin amphitheater, on the southeast side of Democrat 
Mountain. In the bottom of the amphitheater, near its head, is a remarkable 
dike of porphyrite, from 20 to 50 feet wide, which has a straight northeast 
and southwest course and cuts through the narrow wall separating this 
from the amphitheater at the head of English gulch. The main body of 
the dike is a fine-grained, dark-colored rock, more or less impregnated with 
pyrites. Irregularly contained within its mass is a second body of darker 
shade, characterized by inclusions of rounded orthoclase pebbles and large 
crystals or rounded grains of quartz. A specimen of this singular rock is 
shown in Plate VII (p. 86). It is evidently a later eruption within the mass 
of the previously existing dike. Both rocks and their relations are described 
in Appendix A. 

In the Archean rocks here exposed are found all the types previously 
described, and from a face of rock at the head of the amphitheater was made 
the sketch, which is shown in Fig. 1, Plate IV (p. 52), to illustrate the relative? 
ages of the different varieties, normal mica-gneiss being the oldest, which is 
penetrated by the even-grained eruptive granite, and this again in its turn 
crossed by veins of white pegmatite. There are evidences of extensive min- 
eralization, but no ore bodies have been sufficiently opened to afford an op- 
portunity for systematic study. Parallel with the dike in the bed of the 
creek, at the head of the amphitheater, was observed a deposit of galena, 
following the wall of one of the larger pegmatite veins. 

In the mass of Mount Arkansas are many eruptive bodies which could 
not be traced out, although their existence was shown by the numerous 
fragments in the debris. On a northern spur of the mountain two dikes 
were seen, one of a quartz-porphyry, allied to the Lincoln type, the other a 


Ten-Mile and Clinton amphitheaters. The Archean types represented in the 
area embracing these basins are as varied as those which have already been 
described from the adjacent region to the east and south. In Clinton am- 
phitheater there is an unusually large amount of the rudely porphyritic 
granite referred to earlier in this chapter, and in both are numerous dikes 
of eruptive rocks, among which the Lincoln Porphyry and a dark horn- 
blendic porphyrite are the most frequent. These bodies cannot be indicated 
upon the map with satisfactory accuracy, and are for the most part omitted. 
A rhyolitic rock of coarse grain in Ten-Mile amphitheater seems more closely 
related to the Chalk Mountain Nevadite than to any other. 

Bartlett Mountain, which separates the two amphitheaters, has a dike 
of dark-green porphyrite running through its summit and down into the 
Ten-Mile basin. Upon the western slopes of this mountain are large masses 
of white quartz which might at first glance be considered as derived from 
a remnant of the Cambrian strata left on the east side of the Mosquito 
fault, but they are quite homogeneous and have no trace of the granular or 
sandy structure which is found even in the most glassy quartzites; they 
were probably derived from some of the vein-like masses of quartz which 
are found developed on a large scale in the Archean of other parts of the 
Rocky Mountains, and which have been removed by erosion. Abundant 
evidences of glaciation exist in these as in other amphitheaters which have 
been described. 




The central region of the general map is, as has already been seen, 
the region of the greatest dynamic as well as eruptive action. A section 
across the range at Leadville shows, as the result of dynamic action, five 
anticlinal folds and six principal faults. On the east side of the range, as 
already seen, the structure is relatively simple. The beds sloping back to 
the eastward are broken by one main anticlinal fold and its accompanying 
fault, the London fault. On the west of the crest, however, instead of one 
main fault, as in the regions north and south, the continuity of the beds is 
broken up by six principal and several minor faults. 

The map of Leadville and vicinity (Atlas Sheets XIII and XIV) shows 
the most important features of the geology of that region. Its eastern 
border extends to within two or three miles of the main crest, which con- 
sists of Archean rocks capped by easterly dipping Paleozoic beds and in- 
trusive porphyries. For a better comprehension of the description which 
follows, the reader is requested to refer constantly to this map and its ac- 
companying sections. He will there see that its area is divided into a series 
of irregular zones or blocks by the lines of six principal faults having a 
general north and south direction. For purposes of description these have 
received the following names, commencing on the east: 1, Mosquito fault; 
2, Ball Mountain fault ; 3, Weston fault ; 4, Mike fault, with a branch called 
Pilot fault ; 5, Iron fault ; 6, Carbonate fault, with a branch called Pendery 
fault. Besides these there are the following minor and cross faults: 1 , on the 



southern edge of the map, the Iowa gulch cross-fault, which connects the 
Weston and Ball Mountain faults ; 2, the Union cross-fault, which extends 
from the head of Union gulch across upper Long and Deny Hill and joins 
Weston fault in the bed of Iowa gulch ; 3, the Colorado Prince fault, north 
of Breece Hill, a diagonal cross-fault approximately parallel to South Evans 
gulch, which connects Ball Mountain and Weston faults ; 4, on the west 
slope of Breece Hill another cross-fault, the Breece fault, running nearly east 
and west, joining the northern end of Mike fault with Weston fault; 5, a little 
farther west the Adelaide cross-fault, which connects Iron and Mike faults; 
6, at California gulch the southern continuation of Iron fault is formed by 
three different faults : Dome fault, connected with Iron fault by the Califor- 
nia cross-fault, following the line of the gulch, and Emmett fault, which 
connects California fault with Iron fault. Pilot fault, already mentioned, is 
a short north and south fault, crossing California gulch above Mike fault, 
running across the west end of Printer Boy Hill, and joining Mike fault in 
Iowa gulch. The Pendery fault, already mentioned, and the South Dyer 
fault, a cross-fault running eastward from the Mosquito fault along the south 
slopes of Dyer Mountain, raise to seventeen the total number of faults rep- 
resented on the map. 

In ground broken by such a complicated network of fractures and 
subjected since to the enormous erosion which is shown to have taken place 
in this region, it is extremely difficult even for a trained geologist to recon- 
struct ideally the original folds into which the sedimentary beds and their 
included sheets of porphyry were once compressed. As, however, the ac- 
tion of faulting was so intimately connected with that of folding and the 
displacements in many cases pass into simple folds, it is essential, in order 
to obtain a clear idea of the relative position of the different beds below 
the surface and the depths at which they may be found, that one should be 
able to reconstruct in his mind the original folds, and then figure to himself 
the faulting action which has brought the beds into the discordant juxta- 
position in which they are now found on the surface, as shown on the map. 
For this purpose the general structure along certain east and west lines 
will be first described, and after that the present condition of the surface 
and the underground structure, as revealed by shafts in each zone or 


block of ground included between the principal fault-lines, will be described 
in detail. The three cross-sections of the general map which cross the area 
of the Leadville map will serve perhaps best to show the general outlines 
of structure. 

In Section E, Atlas Sheet IX, which is drawn approximately through 
the middle of the map, and which may, therefore, be considered as a type- 
section, the effect of displacement is more prominent than that of folding. 
Its line runs through the southern edge of the town of Leadville itself, 
across Carbonate, Iron, and Breece Hills, passing just north of the crests of 
Ball Mountain and of East Ball Mountain to the summit of West Dyer 
Mountain. Along this line, going from west eastward, the following are the 
main features of folding: In the region under Leadville, or from the western 
edge of the map to the Carbonate fault, a shallow ayncline; under Carbon- 
ate Hill, or from Carbonate to the Iron fault, a second shallow sy ncline ; 
and from Iron Hill eastward, a third ; in all of which the prevailing dip is 
eastward, only a small portion of the easterly edge of the basin having a 
westerly dip. In the region between Iron Hill and Ball Mountain, or, in 
other words, on the western slope of Ball Mountain, the surface is so uni- 
formly covered with Pyritiferous Porphyry that there is no direct evidence 
of any folding, although a slight anticlinal fold might be expected near the 
line of the Pilot fault, from the fact that one exists on its strike both north 
and south. At Ball Mountain is a sharp anticlinal fold, and east of that the 
beds slope back in a monocline to the eastward. The effect of displace- 
ment produced by the faults has been to lift each successive block of ground 
up to the east of the fault, except in the case of a wedge-shaped portion 
included between the Mike and Pilot faults, in which there has been a slight 
downward movement. 

On an east and west line south of this (Section I I, Atlas Sheets XIX 
and XX), the beds of Blue Limestone would be first met about due south 
of the summit of Carbonate Hill, sloping east in a shallow synclinal basin 
and rising again in an anticline whose axis corresponds to the southern 
continuation of the Dome fault. The crest of this fold having been planed 
off bv erosion, the contact would be wanting for something over half a mile, 
and be found at the head of Thompson gulch dipping to the eastward, but 


rising gently as it approached the continuation of the Mike fault. The 
ground east of this fault having been lifted up, the Blue Limestone has 
been in part removed by erosion and would next be found at the Long and 
Deny mine, sloping again eastward as far as Union fault. Beyond this 
fault it has again been removed by erosion, a little remnant only being 
found above the White Porphyry in the uplifted portion adjoining the 
Weston fault. Between this and the Mosquito fault is the arch of an anti- 
clinal fold on which only Lower Quartzite is left. Beyond the Mosquito 
fault erosion has cut down to the Archean rocks, the Blue Limestone being 
next found on the western face of Mount Sheridan, along either of whose 
sides it may be traced, sloping back to the crest of the main ridge. 

On the line of the section north of the first described (Section D, 
Atlas Sheet VIII), which passes through Fryer and Yankee Hills, the 
faulting action is less prominent, owing to the fact that many of the faults 
have in their northern continuation merged into folds. On the north of 
Leadville, extending from the western limit of the map to the eastern edge 
of the town, is the same broad syncline noticed in the first section. From 
here to the western edge of Fryer Hill is a short anticline, from whose crest 
the Blue Limestone has been planed off. It is succeeded by a shallow syn- 
clinal fold under the western half of Fryer Hill, followed by a short anti- 
cline at its crest, while in the gulch back or east of the hill is found the 
rim of a deep synclinal basin which passes under Little Stray Horse Park. 
At the west foot of Yankee Hill the ore-bearing horizon rises to the surface 
and descends to the eastward again just beyond the summit of this hill, the 
crest of the intervening anticlinal fold, into which the northern continua- 
tion of the Iron fault merges, having been eroded off. From this point the 
strata descend to the eastward, rising in a gentle wave near the Great Hope 
mine, but not reaching the surface, and then sloping again eastward until 
they rise on the South Evans anticline or are cut off by Weston fault. 
East of Weston fault, in the region around the mouth of South Evans 
gulch, is another anticline or quaquaversal fold, whose summit has been worn 
away, leaving the outcrops of succeeding beds in a series of concentric 
rings. On the east of this fold the beds slope continuously to the eastward 


at angles of from 15 to 20, a conformable series, extending high up 
into the Weber Grits, being still left uneroded on the summit of Little Ellen 

In Section C, Atlas Sheet VIII, which follows nearly the northern 
boundary of the map, the faults have apparently all been eliminated, and 
the outlines of formations shown on the map owe their form entirely to 
folding and erosion. One broad anticline under the west slope of Pros- 
pect Mountain and a shallow syncline in the Arkansas Valley express the 
broader general features of folding. Near this line, at the mouth of the 
east fork of the Arkansas, are found the westernmost actual exposures of 
Paleozoic rocks within the area surveyed. These consist of beds belong- 
ing to the Lower Quartzite formation, exposed in the bed of the stream and 
in the cliffs south of it, dipping to the southeast. They constitute the most 
definite evidence of the synclinal basin supposed to underlie the town of 


Before proceeding to the detailed description of this region it will be 
well to give a brief outline of the distribution of the various porphyry 
bodies, which form so important an element in its structure and have had so 
great an influence upon its ore deposition. It is first to be observed that 
the features of this distribution have a certain uniformity along northwest 
and southeast lines in approximate parallelism with the line of major strike 
of the sedimentary beds. As by far the greater portion of these bodies arc; 
in the form of sheets either actually or approximately conformable with the 
bedding of the inclosing sedimentary rocks, in cases where explorations 
were insufficient to determine whether they were sheets or transverse dikes 
the former has been assumed to be the case in drawing the ideal portion of the 
various sections, and dikes have been indicated only when actual explora- 
tions have proved that they were coming up directly from below. It may 
readily happen, therefore, in the case of imperfectly explored bodies, that 
future explorations may show the latter form to be more common than lias 
been indicated in the sections. 

white Porphyry. The most important of these bodies is the White Por- 
phyry, which is generally found as a sheet immediately overlying the Blue 


Limestone. As it forms the surface rock over a great part of the area, and 
has hence been subjected to considerable erosion, it is impossible to deter- 
mine its maximum thickness. Its original extent to the southwest is also 
completely unknown, since it, together with the inclosing sedimentary beds, 
has been entirely eroded off from this portion of the area. Along a certain 
zone, moreover, it occurs below as well as above the Blue Limestone. This 
lower body is connected with the upper or main sheet along a line running 
diagonally through the south edge of Fryer Hill, in a southeast direction, 
toward upper Long and Deny Hill and West Sheridan, to the northeast of 
which there are two sheets of porphyry, and to the southwest only one. 
On this line, which is rather a zone than a line and can, in the nature of 
things, be only approximately determined, it is found that there is a. cross- 
cutting sheet of porphyry connecting the two sheets to the northeast with 
the one sheet to the southwest, and the Blue Limestone is in consequence 
found to be split into wedge-shaped and partially-overlapping bodies. The 
greatest development of White Porphyry appears to be a little southwest of 
this zone of cross-cutting, on a line passing through Carbonate, Iron, Printer 
Boy, and Long and Deny Hills, where it attains in places a possible thick- 
ness of 1,000 feet. To the northeast of this zone both sheets thin out rap- 
idly, the lower one before reaching a line running through the forks of 
Little Evans and along the general course of South Evans gulch, and the 
upper one at a little distance beyond this line. Along a line running N. 
50 W. from the saddle east of Ball Mountain to the East Arkansas Valley, 
at the foot of Canterbury Hill, this upper sheet is entirely wanting for short 
distances, coming in again, however, northeast of that line. 

Besides these two main sheets there is a very considerable development 
of White Porphyry along a southeast zone, passing just east of the crest of 
Ball Mountain, where it occurs in the White Limestone and extends down 
to the contact of the Archean. It is a significant fact that in this zone it 
has been proved in two instances to be cutting up through the Archean, in 
the one case in South Evans gulch, near its mouth, and again on the north 
side of Iowa amphitheater. 

Gray Porphyry. Next to the White Porphyry the most important body is 
the main sheet of Gray Porphyry, which, northeast of the Fryer Hill-Slier- 


idan line, is found directly over the upper sheet of White Porphyry in very 
considerable thickness. How great this thickness may have been there is no 
direct means of determining, since, except on Prospect Mountain (where no 
shafts have been sunk to any great depth), no sedimentary rock remains 
above it to give its upper limits. Its greatest observed thickness is 420 feet 
in the Independent shaft. What was the original extent to the southward 
of this Gray Porphyry sheet before any portion of it had been removed 
by erosion, there is also no means of determining. At present it extends 
but little beyond the median line of the map. Its source must probably be 
looked for to the northward, beyond the limits of the map, since in that 
direction it passes into Lincoln or Eagle River Porphyry, of which it seems 
to be merely a decomposed variety. 

Pyritiferous Porphyry. Next in importance in point of superficial extent, 
and possibly of greater importance in its bearing on the ore deposits of the 
region, is the Pyritiferous Porphyry. The main sheet of this porphyry, 
which covers the lower slopes of Breece Hill, seems to be a stratigraphical 
replacer of the Gray Porphyry, which however, along the line of the Breece 
fault, it overlaps. It may therefore be supposed to have been in point of 
time a later intrusion. As is shown in Section G, one of the vents, and 
possibly the sole vent, probably existed beneath California gulch Its ex- 
tent to the north and east could not have been much greater than at present. 
To the south and west, however, it may have covered a considerably larger 
area immediately above the White Porphyry. Of the upper sheets in the 
Weber Grits no opinion can be formed, so completely has all trace of this 
formation been removed by erosion. It is perhaps fair to assume that it ex- 
tended to the south as far as the present crest of Long and Derry Ridge, 
and to the westward over Irpn Hill, and, possibly, as far as Carbonate Hill. 

other porphyries. Of the other bodies of porphyry, the most important in 
their bearing upon the ore deposition of the region are those which are 
essentially of the same rock as the main sheet of Gray Porphyry, though 
having no apparent connection with it. The most extensive sheet of this 
rock is found under Iron and Carbonate Hills, near the base of the Blue 
Limestone, and cutting up across this horizon to the westward; various irreg- 
ular, dike-like bodies found in the different mines are, doubtless, offshoots 


from this sheet. A small sheet is found above the Blue Limestone on Iron 
and Dome Hills. A large body, probably coming up from below, occurs on 
the southeast face of Yankee Hill, extending across Adelaide Park. Several 
small sheets are found in the White Limestone on the north end of Iron Hill 
and in California gulch, and three well-developed dikes cross Printer Boy 
and Long and Derry Hills. 

On the northwest slope of Printer Boy Hill the Printer Boy Porphyry 
forms an important mass ; in Iowa gulch is the Green Porphyry, under the 
White Limestone; and on Long and Deny Ridge, the Josephine Porphyry, 
above the Blue Limestone. Mount Zion Porphyry, which is closely allied 
to the White Porphyry, forms a body of great thickness in the Weber Grits 
on Prospect Mountain, and is found also in Evans gulch, but seems to be 
simply a local occurrence which reaches its greatest development beyond 
the limits of the map, and has apparently had little or no influence upon 
the ore deposits of the district. 

In what follows will be given in detail the various facts upon which 
the geological deductions represented on the map and sections have been 
founded, which will probably prove of interest only to those who wish to 
verify these deductions on the ground or examine into their soundness. 
For convenience of description the region will be divided into the areas 
naturally blocked out by the lines of the principal faults, and these will be 
treated in topographical order, proceeding from the east westward, and in 
each block from the south northward. 


Between Mosquito fault and the crest of the range is a considerable 
area, occupied principally by Archean exposures, whose description prop- 
erly belongs to that of the Leadville region, although only a small por- 
tion of it is actually shown in the extreme southeast corner of that map. 
In it, immediately below the main crest, are the two great glacial amphi- 
theaters in which headed the glaciers that carved out Evans and Iowa 
gulches, and which offer the best opportunities in the immediate vicinity of 
Leadville for the study of the relations of the ancient crystalline rocks and 

of the eruptive bodies that have been intruded through them into the over- 
MON xn 14 


lying Paleozoic formations. Their scenery is moreover of an imposing and 
Alpine character that would hardly be expected from the somewhat tame 
appearance of the immediate vicinity of the city itself. For this reason a 
view of the more picturesque and instructive of the two, that of Iowa am- 
phitheater, has been chosen for the frontispiece of this volume. 

Frontispiece. Iowa amphitheater, as will be seen on the Mosquito map, 
is a bowl-shaped depression some 2,500 feet deep, with three main branches 
or subsidiary amphitheaters extending up to the north, northeast, and south 
between the bounding peaks, Dyer Mountain, Mount Sherman, and Mount 
Sheridan. The view given in the frontispiece is the reproduction of a not 
altogether satisfactory photograph taken from a point on the north side of 
Iowa gulch, at the foot of the steeper southern slope of East Ball Mountain. 
The spur in the foreground, on the right, is a portion of the north slope of 
West Sheridan, formed of Archean rocks capped by Lower Quartzite ; that 
on the left is the south spur of East Ball mountain, along which runs the 
Mosquito fault; while the background is formed by the west face of Mount 
Sherman, an almost vertical wall 2,400 feet in height. On this wall the 
up per 1,400 feet are occupied by the main sheet of White Porphyry, over- 
lying the lower Paleozoic beds, in which are also several minor sheets of 
the same rock, too small to be indicated except in a general way on the 
Mosquito map ; and the base of the cliff is formed by Archean rocks. In 
the view the horizontal lines of the stratified beds can be readily distin- 
guished from the somewhat gothic forms of weathering of the great mass 
of porphyry above, but the lower portions of the cliff are almost entirely 
hidden beneath talus slopes of debris, through which only here and there 
projects a portion of the Archean granite. 

In the granite exposures on the south bank of Iowa Creek, a little 
above the point from which the frontispiece view is taken, are the best 
examples of glacier action on rock surfaces in this region. The granite 
bosses here have a gentle slope to the east and are steep on the west, the 
whola upper surface of the rock being beautifully polished, grooved, and 
striated, and the lines being parallel with the direction of the gulch. These 


striation lines are particularly fine on the surface of the large feldspar crys- 
tals, where, when closely examined, they are seen to resemble the parallel 
lines of a steel engraving. 

Mosquito fault. The average course of Mosquito fault, which forms the 
western boundary of this area, is magnetic north or north 15 east. From 
the point where it branches off from the Weston fault, in the bed of Empire 
gulch, it runs across Upper Long and Derry Ridge at the foot of the steep 
face of West Sheridan, through Iowa gulch, along the west face of East 
Ball Mountain, and through the narrow saddle on the ridge between West 
Dyer Mountain and Little Ellen Hill into Evans amphitheater, which it 
crosses diagonally, near if not actually through the shaft of the Best Friend 
mine, to the foot of the zigzag road descending from Mosquito pass. Ow- 
ing to the absence of shafts in the region, its location can only be determined 
by actual rock outcrops, and where these" are obscured by debris it may 
vary a little one way or the other from that given on the map. Its throw, 
which varies somewhat at different points, may be taken at an average of 
4,000 feet. 

Minor faults. Of the minor or cross faults in this area only one, the 
South Dyer fault, appears on the Leadville map. By its movement, which 
was an upthrow to the north, a fragment of the Lower Quartzite beds, with 
an included sheet of White Porphyry, has been left on the southwest spur 
of East Ball Mountain, where it forms a shoulder half-way down the slope 
and is entirely surrounded by Archean outcrops. Beyond the line of the 
map it crosses the south foot of Dyer Mountain, where a dike of White 
Porphyry cuts through the Archean on its probable continuation and joins 
the Sheridan fault in the bed of Iowa gulch. The Sheridan fault runs at 
right angles to the former in a southwest direction across .the saddle between 
Mount Sheridan and West Sheridan, and is supposed to join Weston fault 
in the north head of Weston gulch. Its movement is a slight upthrow to 
the east, and the combined displacement of these two faults explains the 
existence of a singular triangular-shaped mass of White Limestone and 
Lower Quartzite at the entrance to the north branch of Iowa amphitheater, 
in the very bed of the gulch. As the normal continuation of these beds is 
found high up on the face of the surrounding mountains, it might, seem at 


first to have been dropped down here by a sudden sinking of the ground. 
Nearly parallel with the South Dyer fault is the Dyer Mountain fault, whose 
presence is indicated by a slight discrepancy in the stratified beds at the head 
of Dyer and North Iowa amphitheaters. Its extent as well as its movement 
is apparently small, as it could not be traced beyond these valleys in either 
direction, and in the Dyer amphitheater is shown only on the east wall, 
there being apparently no break in the lines of stratification on the West 
Dyer Mountain side. The amount of displacement caused by these two 
faults is shown in Section M, Atlas Sheet X. 

West Sheridan. West Sheridan Mountain, which is in point of fact 
simply a Y-shaped spur extending out westward from the main Mount Sher- 
idan, has, as it were, three summits, two of which are capped by the remains 
of the White Porphyry sheet, which here separates the Blue Limestone from 
the White. The remainder of the crest of the ridge is formed by beds of 
White Limestone, under which is a fringing outcrop of Lower Quartzite. 
In those on the north and west slopes are several small bodies of White 
Porphyry. An estimate of the thickness of White Limestone and Lower 
Quartzite on the western face of the south and north spurs of West Sheri- 
dan, respectively, gave 250 and 275 feet, the difference being accounted ior 
by included sheets of White Porphyry. 

Dyer Mountain. Dyer Mountain, as shown in Section M, whose line runs 
from the summit of the mountain southward through the spur represented 
in the photograph, is composed of the following beds in descending series: 

Sacramento Porphyry. 
Weber Grits." 
White Porphyry (main 
Blue Limestone. 

White Limestone. 
Lower Quartzite. 
White Porphyry. 

The main sheet of White Porphyry is the cap-rock of that portion of 
spur shown in the frontispiece. The lines of stratification on the face of 
the spur toward the observer, dipping gently to the north toward the head 
of Dyer amphitheater, belong to the Lower Quartzite and to the underlying 
bed of White Porphyry, which is here two hundred to three hundred feet in 
thickness. On the south face of this spur, toward Iowa gulch, in the Archean 
apparently near or on the line of the South Dyer fault, is an irregular out- 



crop of White Porphyry, somewhat in 'the form of a dike, parallel to the fault, 
but with ramifying branches extending in various directions. This body 
is interesting as being one of the few cases where the White Porphyry 
could be seen to have been directly erupted through the Archean, and is 
very probably the source from which the lower sheets of this rock have 
spread out between the lower Paleozoic beds below the horizon of the 
Blue Limestone ; it seems hardly of sufficient size, however, to account for 
the. immense thickness of the main sheet of White Porphyry above that 
horizon, and whose source, as already shown, is supposed to exist in the 
White Ridge near the head of Four- Mile Creek. 

' On the east wall of Dyer amphitheater, in the upper part of the White 
Limestone near the Parting Quartzite, are the deposits of the Dyer mine, 
from which the mountain has derived its name. This mine is one of the 
earliest discoveries of the district, antedating by many years that of the 
carbonate mines, but owing to its great altitude and difficulty of access it 
has been but intermittently worked. A section measured along a steep 
hillside, with a slope of 32, just south of the Dyer mine, gave the follow- 
ing thicknesses: 

Lower Carboniferous 



( Blue Limestone : 

\ Dark blue, weathering black, with black chert 150 

f Parting Quartzite: 

Sandstone and silicious limestone . . 10 

White Limestone : 

Thin-bedded, bluish limestone 35 

Light-blue limestone, couchoidal fracture, passing. 

into pinkish, clayey material 15 

Gray, semi-crystalline limestone 40 

Sanely lime tone, with some sandstone 30 

White, silicious limestone 10 



Cambrian . 

Lower Quartzite : 

Red-cast beds 10 

Reddish brown qnartzites 50 

White, saccharoidal quartzite 100 


White Porphyry, 200 feet. 
Archean Granite 


Just below the Dyer mine a bed of limestone of a light steel-blue color 
is singularly changed into a light-pink, clayey material, so different in ap- 
pearance from the unaltered rock that a partial analysis of the two was 
made in order to determine the chemical change that had produced this 
appearance. The following figures were obtained: 

Unaltered limestone. Altered limestone. 

Lime 20.31 19.21 

Magnesia 10.35 0.58 

Alumina and iron 0.23 5.23 

Insoluble 31.27 34.56 

From which it is seen that the alteration consists mainly in the removal 
of a portion of the soluble bases and a consequent relative increase in the 
proportion of silica. It also shows that a very essential change in the 
physical character of a rock may be made by the action of percolating 
waters, with very little actual chemical change. 

The break in the beds north of the Dyer mine, caused by the move- 
ment of South Dyer fault, is very evident in the Blue Limestone, but cannot 
be traced much below that horizon. On the west wall of the Dyer amphi- 
theater the beds slope up the face of West Dyer Mountain in an unbroken 
line, showing no trace of the fault; the main sheet of White Porphyry 
which forms the saddle between Dyer and Evans amphitheaters thins out very 
rapidly to the northwest, and on the face of West Dyer Mountain shows an 
outcrop of only about ten feet, the summit of the peak being formed by a 
few remaining beds of Weber Grits. 

Evans amphitheater. The basin at the head of South Evans gulch, as 
well as the main Evans amphitheater, shows mainly outcrops of Archean 
rocks, those of the Weber Grits, which adjoin them west of the fault line, 
being generally covered by debris. The wall of Mount Evans facing the 
amphitheater presents similar conditions to the wall at the head of Iowa 
gulch, namely, an eruptive mass underlaid by horizontal stratified beds, 
and the same strong contrast in their weathered forms, the difference being 
that in this case it is the Sacramento instead of the White Porphyry that 
forms the intrusive mass. In a shallow ravine on this wall just south of 
the Mosquito pass there is a slight break in the continuity of sedimentary 
outcrops, caused by a small cross-fault with a slight upthrow on the north. 


East Bali Mountain. The crest of what is known as East Ball Mountain, 
which is in reality only a spur of West Dyer Mountain, is capped by Lower 
Quartzite, with a sheet of White Porphyry between it and the underlying 
Archean. This sheet is evidently the same which has already been noticed 
on the south spur of Dyer Mountain ; in the recess of Dyer amphitheater, 
however, it must cut partly across the Lower Quartzite, since on the west 
wall of the amphitheater there is a considerable thickness of Lower Quartz- 
ite below the White Porphyry. 


Bali Mountain fault. The direction of the Ball Mountain fault is some- 
what to the west of north, nearly parallel with that of the Weston fault, 
and therefore convergent with the Mosquito fault, which it joins on the crest 
of the Upper Long and Deny Ridge at the foot of the steep slope of West 
Sheridan. From here it runs in a direct line across Iowa gulch, through 
the top of Ball Mountain, and bends sharply to the west on its northern 
slope, passing through the End Squeeze or Cleopatra shaft (F-12). 1 Its 
movement is defined here by the Fat Purse (F-17), which is in Weber Grits 
on the west of the fault, and the John Mitchell (E-ll), which is in Lower 
Quartzite on the east; it then runs northward across South Evans gulch, 
through the Nevada tunnel, which has been driven nearly three hundred 
feet on its line, and just west of the Seneca shaft, which is in the White 
Limestone. Farther north its existence is shown only by the widening of 
the outcrop of Weber Shales and by a slight discrepancy in the outlines 
of the body of Mount Zion Porphyry in Evans gulch. Its movement of 
displacement is an upthrow on the east, which has a maximum at the south- 
ern end, or in Iowa gulch, of 2,250 feet, and gradually decreases to the 
northward, being only a few feet in Evans gulch and disappearing entirely 
in Prospect Mountain. 

Prospect Mountain Ridge. On the spur from Prospect Mountain west of 
the Prospect amphitheater, which is on the line of the fault, there is a slight 
variation in the regular easterly dip of the Weber Grits, which suggests 

1 The letters aud numbers following the names of shafts indicate, respectively, the square aud 
the uumber within that square, by means of which the shaft may be fouud ou the Leadville map. 


that the influence of the fault has produced a slight anticlinal fold. Pros- 
pect Mountain, from its summit eastward to the foot of Mosquito pass, is 
made up of coarse sandstones and micaceous shales of the Weber Grits 
formation, which dip a little north of east. 

Little Eiien Hiii. The same beds are found to extend through the main 
portion of Little Ellen Hill and across the upper part of South Evans gulch, 
and outcrops where visible have a prevailing dip of 20 to the eastward. 
The lines of structure in a series of beds of such uniform composition are 
difficult to trace in a country where the surface is so much obscured as here. 
It is possible, therefore, that the eastern ends of Sections A, B, C, and D, 
which pass through this region and have been constructed somewhat theo- 
retically, give too great a thickness for this formation; in other words, place 
the ore horizon at too great a depth below the surface, since the structure 
lines obtained from other portions of the region, where definite data are 
more frequent, show no such extent of regular slope, but much more fre- 
quent waves or folds. Such, however, have not been indicated here, as in 
the absence of definite data they would be purely imaginary. 

Eruptive dikes. In this area are several outcrops of eruptive bodies, 
which apparently belong rather to the dike type than to that of intrusive 
sheets. Two of these occur on Prospect Mountain ridge, the easternmost of 
which is a coarse-grained quartz-porphyry, with large orthoclase feldspars, 
resembling the Lincoln Porphyry; its feldspars are partly reddish and 
partly light green, the coloring being due to iron oxide on the one hand, 
or to light-green mica as an alteration product on the other. The western 
of these dikes is a fine-grained, dioritic-looking rock, similar to that found 
in the Arkansas Valley between Indiana and Bird's Eye gulches and at the 
heads of these gulches. On the north slope of Little Ellen Hill is an out- 
crop of the same coarse-grained porphyry that is found in the eastern 
dike. In the bed of Evans gulch, above the Virginius mine and extend- 
ing up some distance on the north side of the gulch, is an eruptive mass of 
rather irregular form, whose outlines are somewhat obscured by surface 
accumulations. It belongs, as well as could be ascertained from the par- 
tially decomposed specimens obtained, to the Mount Zion type of porphyry. 


Coal in Weber Shales. The carbonaceous beds of the Weber Shales series 
are unusually well developed in this region and often contain considerable 
impure anthracite. The greatest developments of this coal are found in the 
Ellesmere (B 2) and Little Providence (C 8) shafts, in the former of which 
it is said to have a thickness of eighteen inches and in the latter of seven feet. 
The coal, however, has thus far proved too impure to be of economic 
value. Similar beds of coal have been observed by the writer at what is 
very probably nearly the same horizon in the Pancake Mountains west of 
White Pine, a short distance from Argenta on the Central Pacific Railroad, 
and some 30 miles north of Elko on Coal Creek, in Nevada. Explora- 
tions at all these localities have, however, failed to develop any workable 
beds of good coal. In a region like Colorado, therefore, where the Creta- 
ceous formations which are known to contain abundant beds of excellent 
coal are so widely developed, it seems scarcely advisable to spend much 
labor in searching for coal at this lower horizon. The name "Carbon- 
iferous," which was given to this formation in the early days of geology, 
when it was supposed to be the only coal-bearing horizon, is a practical 
misnomer in the Rocky Mountain region, and apt to mislead the untechnical. 

Blue Limestone. The outcrop of Blue Limestone from the point where 
the Ball Mountain fault crosses South Evans gulch, just below the Seneca 
shaft, follows up the north bank of the gulch and then bends to the south up 
Alps gulch to the saddle between Ball Mountain and East Ball Mountain. 
Its existence is proved by explorations of the Little Rische (G 6), Little 
Ellen (G-5), Lulu(G-4), Izzard (Gr-3), Gnome (G-2), Wall Street (G-l), 
Dauntless (C 13), and Alps shafts, which have cut through the overlying 
White Porphyry to the contact. In the Little Ellen alone has any con- 
siderable body of ore been discovered at the contact. Iron vein material 
of considerable thickness has been found on the contact in the workings of 
the Alps group of mines, but as far as known little rich ore has been devel- 
oped. The White Porphyry is here very thin, and at the head of Alps gulch 
disappears entirely, coming in again on the south side of the ridge. From 
this saddle the outcrop of the Blue Limestone sweeps round to the eastward 
to the Black Hawk shaft. This shaft passed through 80 feet of White Por- 
phyry and a little black shale before reaching the Blue Limestone. Beyond 


this the Blue Limestone is cut off by the Mosquito fault at the sharp spur run- 
ning out southwest from East Ball Mountain toward Dyer gulch. Actual 
outcrops of Blue Limestone and Parting Quartzite, dipping steeply to the 
east, are found on the saddle east of Ball Mountain. The summit of Ball 
Mountain, from the saddle westward to the fault, is formed of White Por- 
phyry, of which a thick body here separates the Parting Quartzite from the 
White Limestone. 

The structure of the area between the crest of Ball Mountain and the 
south slope of Ellen Hill below the outcrop of Blue Limestone is some- 
what obscure; but the explanation presented on the map, viz, that of a 
quaquaversal or anticlinal fold in part cut off by the Ball Mountain fault, 
is one which best fits the following observed facts. 

On the north slope of Ball Mountain, immediately under its crest, can be 
traced outcrops of White Limestone and of a portion of the Lower Quartz- 
ite beneath it, The prominent ridge which extends out on the steep slope 
towards the valley of South Evans consists entirely of fragments of quartzite 
derived from the above-mentioned outcrop. .Shaft F-6, however, which 
has penetrated this covering of debris, shows that the underlying rock is 
White Porphyry. The John Mitchell shaft, west of this, has gone through 
another body of Lower Quartzite and a second underlying White Porphyry 
to granite. North of the John Mitchell, at the base of the hill, is a small 
outcrop of granite, with a little white quartzite resting on it. Still north of 
this are the Ocean and Seneca shafts, the former in Lower Quartzite, the 
latter in White Limestone, dipping to the northward. It seems, therefore, 
that the White Porphyry is here splitting the Lower Quartzite into several 
distinct bodies, and it may naturally be inferred that somewhere in this re- 
gion it has been intruded into the Lower Quartzite from the underlying 

The Nevada tunnel discloses a body of White Limestone, resting on 
quartzite, east of the fault, which dips at 45 to the north. This dip is some- 
what abnormal to the quaquaversal structure deduced from observations in 
other shafts of this region, but its proximity to the fault may account for 
the irregularity. 


South slope of Ball Mountain. On the south slope of the Ball Mountain Ridge, 
towards Iowa gulch, the White Limestone is also split into three distinct 
sheets by intrusive masses of White Porphyry. They might perhaps be 
considered to be simply caught up and included in the porphyry body. 
One or more of these sheets can be traced on the upper slope of Long 
and Deny Ridge, in the angle between Ball Mountain and Mosquito faults. 
South of the crest of Ball Mountain the Emma tunnel (C-10) is run on 
the contact of White Porphyry and White Limestone, following some iron- 
stained vein material. West of this and adjoining the fault, the Lower 
Quartzite outcrops under the White Limestone, dipping at an angle of 50 
to the east. Another portion of the Lower Quartzite is found adjoining the 
fault on the shoulder south of Ball Mountain, and in the bed of Iowa gulch 
erosion has exposed the full thickness of the Lower Quartzite, with a -small 
area of Archean rocks next to the fault on the east, the quartzite dipping 
east at an angle of 18. 

The distribution of the White Porphyry bodies in this area is rather 
exceptional, as they are principally developed in the lower horizons, form- 
ing several sheets within the Lower Quartzite and White Limestone, while 
the bed above the Blue Limestone is comparatively thin, and at one point 
entirely wanting. It might be inferred from this that near here is one or 
more of its vents or points where it has been intruded through the Archean 
into the overlying Paleozoic beds. 


This area presents a still more complicated structure than the one just 
described, owing, first, to the existence of a well-defined anticlinal or qua- 
quaversal fold, the South Evans anticline ; second, to the disturbance pro- 
duced by the Iowa gulch and Colorado Prince cross-faults, which run trans- 
versely across the area ; third, to the peculiar movement of displacement of 
the Weston fault, which has the normal upthrow to the east at its northern 
and southern extremities, but in the intermediate region has partly a reversed 
throw or to the westward, and in one portion no displacement at all ; and, 
fourth, to the branching of the Weston fault at its southern end, by which 


part of its movement of displacement is distributed to the Union fault. The 
prevailing dip of the formations is to the east, except in the vicinity of the 
South Evans anticline. 

Weston fault. This fault is approximately parallel with Ball Mountain 
fault, and follows the same general direction that it had in the area already 
described outside the limits of the Leadville map. From its intersection 
with Mosquito fault in Empire gulch, it crosses the Long and Derry Ridge, 
near the foot of the steeper slope of the Upper Long and Deny Hill, and 
descends into Iowa gulch just east of the Ella Beeler tunnel (E-7), where 
it is joined by the Union fault; it runs thence diagonally up the southwest 
slope of Green Mountain, a little east of the North Star (E-23) and Alta 
(E-22), and crosses the head of California gulch between the Tiger shaft 
(E-24) and the Ella tunnel (F-39). From here up the slope of Breece Hill 
its position cannot be exactly defined, owing to the fact that Pyritiferous 
Porphyry forms the reck surface on either side. In the ground of the 
Highland Chief No. 2, however, it passes between two shafts of that claim 
(F-40 and F-41), the former of which is in the Weber Grits and the 
latter in Pyritiferous Porphyry, and just east of its main shaft (F 59) ; it 
then follows the crest of the ridge to its north point and down its steep north 
slope between the Chemung tunnel on the east and the Fenian Queen on the 
west, along the line of the west fork of Lincoln gulch to Big Evans gulch, 
which it crosses just above the month of Lincoln gulch. On the south 
slope of Prospect Mountain its movement of displacement becomes \ery 
slight, and is proved only by the discrepancy in the position of the divid- 
ing line between Weber Shales and Gray Porphyry, as shown in the Still- 
well (K-ll), La Harpe (K-12), and other shafts in Little Evans gulch 
on the one side and in the Mary Able (K-35) on the other. 

The movement of displacement of this fault is quite remarkable, being 
an upthrow to the west of about six hundred feet in Iowa gulch and about 
the same amount of displacement reversed, the upthrow being on the east 
side, near the mouth of Lincoln gulch, opposite the South Evans anticline. 
This movement becomes null between these two points somewhere in the 
neighborhood of the Yates shaft (F-6(i) on Breece Hill. 


Iowa fault. The Iowa cross-fault runs east and west along the foot of 
the cliff, on the north face of Upper Long and Deny Hill, and connects Wes- 
ton and Ball Mountain faults. The shafts and tunnels between it and the 
bed of the gulch are either in Weber Grits or Pyritiferous Porphyry, while 
Archean granite forms the cliff above it on the south. The estimated dis- 
placement of this fault is an upthrow of 2,700 feet to the south. It might 
perhaps be better considered a downthrow to the north, since that portion 
of the area which immediately adjoins it on that side is in the abnormal po- 
sition of being relatively lower than the corresponding block of ground on 
the west of Weston fault. 

The uplifted block of ground inclosed by Iowa and Weston faults 
consists of Archean rocks, principally coarse red granite, with a narrow 
strip of Lower Quartzite resting on them along the crest of upper Long 
and Deny Hill. This quartzite is apparently the crest of a shallow north 
and south anticlinal fold, now almost entirely eroded away. The curve 
of the strata can be readily seen on the cliff from Iowa gulch; at the 
western end, toward the fault, the dip steepens to 30. At the eastern end, 
on either side of the ridge, is an outcrop of a much decomposed, coarse- 
grained quartz-porphyry, which' apparently forms a sheet between the 
quartzite and underlying granite. 

Southwest slope of Bali Mountain. From Iowa fault north ward, across Iowa 
gulch and up the southwest slope of Ball Mountain, the surface is mainly 
covered by Pyritiferous Porphyry, with occasional outcrops of the sand- 
stones and shales of the Weber Grits. The sedimentary beds all have a. 
gentle dip to the northeastward and are separated by intervening porphyry 
sheets into three distinct series, the lowest of which is classed as Weber 
Shales, although black shales are found to a greater or less extent through 
all the beds. 

This lowest series, which crosses Iowa gulch opposite the Ella Beeler 
tunnel and extends part way up the slope of Green Mountain, comes in 
juxtaposition with the White Porphyry beyond the fault to the west, and 
is overlaid by a thin body of Pyritiferous Porphyry, which is cut in a 
tunnel (E-21) on the slope of Green Mountain. 


The base of the Weber Grits, consisting of quartzite, conglomerate, 
and shale, is shown on the south side of Iowa gulch in the Little Hercules 
tunnel (E-6), and on the north side in the Black Cloud shaft, which cuts 
through it into the underlying porphyry. On Green Mountain the Hoo- 
sier (E-19) and adjoining (E-20) shaft are sunk in it, and the Equator 
(E-17) tunnel runs on the contact breccia material between it and a sec- 
ond sheet of Pyritiferous Porphyry above. The outcrops of this body, 
consisting of iron-stained decomposed porphyry, can be easily traced along 
the slope of Iowa gulch, but the underlying Weber Grits are obscured by 
debris. South of the gulch it is shown in the Mount Carbon tunnel and on 
Green Mountain in the Tiger shaft and in the Ontario and Bloomington 
(E-9) tunnels. The former passes at 200 feet from its mouth into mica- 
ceous sandstones and shales of the second sheet of Weber Grits, while the 
shaft (E-18) shows breccia material between the sandstones and the under- 
lying porphyry. 

The outcrops of the second body of Weber Grits sweep round the 
upper part of Green Mountain, where the Green Mountain and Lawrence 
(E-10) 1 shafts have reached it after crossing the lower part of the upper 
body of Pyritiferous Porphyry; it widens out in the vicinity of the Little 
Frank (D-2) shaft and the Alleghany and Pine Forest tunnels, and again 
thins to thirty or forty feet as it crosses Iowa gulch to the Iowa fault below 
shaft D-4. Above this body of Weber Grits the main sheet of Pyritifer- 
ous Porphyry extends up to the crest of Ball Mountain and east to the 
fault, broken only by isolated outcrops of Weber Grits, apparently repre- 
senting fragments of this formation caught up in the mass of the porphyry 
at the time of its intrusion. Such a fragment, consisting mainly of black 
shales, is cut in the Silver Queen tunnel on the hill slope above the Pine 

The weathered surface of the Pyritiferous Porphyry in general shows 
no pyrites, but only the cavities from which its crystals have been dis- 
solved out. The old Mariner tunnel, above the Silver Queen, has been run 
125 feet from the surface in porphyry thus decomposed, and at the mouth 
of the former is a deposit of needles of spruce, cemented together and partly 

'Wrongly named Leaven worth on the index sheet of the Atlas. 


replaced by limonite resulting from the leaching out of the pyrites. The 
Weber sandstones at the contact with the porphyry are often brecciated 
and so impregnated with pyrites as to be scarcely distinguishable from the 
porphyry. To understand the distribution of the various bodies of this 
porphyry and their supposed continuation below the limits of exploration 
it will be necessary to refer to Section G, Atlas Sheet XX. According to 
this it will be seen that there are three distinct sheets of porphyry above 
the Weber Shales, while a fourth occurs beyond Weston fault, between the 
Weber Shales ?nd White Porphyry, which connects with a fifth body 
found in California gulch, where it crosses the White Limestone and White 
Porphyry along the cross-cutting zone already mentioned. It is supposed, 
as shown in the section, that it is in this vicinity that the porphyry came 
up through the Archean. 

Northwest slope of Bail Mountain. The bodies of Pyritiferous Porphyry thin 
out towards the north, and on the upper part of Breece Hill, or the north- 
west slope of Ball Mountain, all except the upper one disappear before 
reaching the Colorado Prince and Ball Mountain faults. No trace of Pyri- 
tiferous Porphyry has yet been found east of the latter fault. To the north 
the lower sheet is stratigraphically replaced by the main sheet of Gray 
Porphyry, since farther west they are each underlaid by the main body 
of White Porphyry. In the few cases where underground explorations have 
disclosed the relations of the Pyritiferous and Gray Porphyries the former 
is found to overap the latter. 

In this region the beds whcih have been assigned to the Weber Shales 
horizon are found to have a much larger proportion of sandstone than the 
corresponding beds on Little Ellen Hill, but in either case the data are 
somewhat meager, as no shafts are so situated as to afford a complete or 
continuous section. The Antelope shaft found 100 feet of white quartzite 
immediately above the Gray Porphyry. The Quandary shaft found mi- 
caceous sandstone ; the Garbutt, sandstone and shale ; and the shafts of 
the Ontario (F-50 and F-51), a coarse sandstone. The Capitol (F-57) 
and the Highland Queen (F-56) shafts passed in depth into a body of 
quartz-porphyry of different character from the Pyritiferous Porphyry. 
It were too long to enumerate all the other shafts on this slope, the infor- 


mation obtained from which is sufficiently indicated on the map and 
sections. The Gray Porphyry sheet under the Pyritiferous also thins out 
to the eastward and cannot be traced beyond the Colorado Prince fault. 

Colorado Prince fault. The movement of displacement of the Colorado 
Prince fault is an upthrow to the southwest, which is about two hundred 
feet in the middle and reaches a maximum at its junction with the Ball 
Mountain fault and a minimum at that with the Weston fault. By this 
movement the southern portion of the South Evans anticline has been cut 
off and leaves, along the cliff at whose foot the fault runs, a succession of 
beds dipping generally to the southward; to the faulting and consequent 
exposure of the edges of these beds the steepness of the cliff is doubtless due. 

The White Porphyry sheet immediately over the Blue Limestone is 
here very thin and in places entirely wanting. The Chemung tunnel <it 
the western edge of this area, near the Weston fault, runs in a southeast 
direction on the contact between the Gray and White Porphyries, and, 
cutting across the latter, passes into the Blue Limestone. The Highland 
Chief and adjoining shafts, a little east, of this, pass through the Gray Por- 
phyry directly into silicious vein material, which is the replacement here 
of the Blue Limestone. North of this the Eliza No. 2 (K-3) strikes Blue 
Limestone, the Little Alice (K-2) Parting Quartzite, and the Eliza No. 1 
(G 58) White Porphyry after passing about twenty feet of Wash, the last 
reaching White Limestone at a depth of 180 feet. (Shaft No. 2 (G-51) of the 
Miner Boy finds a small body of black shales directly above the limestone, 
between it and the overlying Gray Porphyry. The Uncle Sam shafts (F 32 
and F-33), at the east end of Idaho park, find vein material between Gray 
Porphyry and Blue Limestone. The Rattling Jack and Little Johnny, still 
farther east, find a sheet of White Porphyry between the Gray Porphyry 
and Blue Limestone, with vein material at its contact with the latter. The 
body of White Porphyry cut in the Little Alice and Eliza No. 1 shafts is 
evidently of limited extent, as it has not been seen in any other workings. 

The White Limestone and Lower Quartzite are cut by the various 
shafts and tunnels of the Black Prince, Miner Boy, and Colorado Prince 
claims, the tunnel of the last running south through a body of White 
Porphyry between the granite and Lower Quartzite and in close proximity 


to the Colorado Prince fault. A more detailed description of the structure 
in the immediate vicinity of these mines will be found in Part II, Chapter V. 

In the east fork of Lincoln gulch, according to Mr. Jacob, the Boulder 
incline is sunk on the line of the Colorado fault at an angle of 45 to the 
north, with granite in the roof and White Porphyry in the foot-wall; while 
the Cumberland shaft, a short distance east, is sunk 150 feet in White Por- 
phyry. The evidence of the former would prove, therefore, that the Colo- 
rado Prince fault at this point is a reversed fault, viz, that the upthrow is 
on the hanging-wall side, instead of, as is usually the case, on the foot- wall; 
and the fact that the latter was sunk to so great a depth in White Por- 
phyry without reaching granite Avould be explained by the nearly vertical 
position of the sheet. This explanation, which considers the White Por- 
phyry as an interbedded sheet, is supported by the apparent continuity of 
White Porphyry north of the Colorado Prince fault, around the outcrop of 
Archean. It seems possible, however, that these shafts may be sunk in the 
actual channel through which the porphyry came up across the Archean. 
The Fitchburg incline, on the south side of Lincoln gulch, opposite the 
Boulder, was run down on the contact of Lower Quartzite and White 
Limestone, which here dip 20 to the southwest. 

South Evans anticline. The granite body which forms the crest of South 
Evans anticline, extending from the north fork of Lincoln gulch to the 
mouth of South Evans, is shown wherever prospecting has stripped the 
rock of the overlying soil; also, by the Caledonia (G-59) and Slim Jim 
(G-46) shafts and by the Silver Tooth bore-hole (G-45) This bore-hole 
was sunk 314 feet, cutting in its passage downwards 49 feet of White Por- 
phyry, probably a small dike within the granite. The granite in the Cale- 
donia is red and coarse grained ; that from the bore-hole, compact and fine 
grained. The White Cloud shaft (K-15) on the. west side of the fold in 
Lincoln gulch was sunk in the Lower Quartzite, which also outcrops near 
the road, showing a western dip. A shaft (K-14) on the north side of 
Evans gulch has found quartz-porphyry directly under the Wash. The 
Hoosier Girl (G-44), on the east, is in Lower Quartzite, which must be 
a portion separated from the main body by the lower sheet of White Por- 
phyry. This lower sheet of White Porphyry forms the western point of 



Little Ellen Hill, between South Evans and Evans gulches; it is coarser 
grained than the normal rock and contains numerous quartz crystals. The 
successively higher horizons of Lower Quartzite, White Limestone, Blue 
Limestone, White Porphyry, and Weber Shales are crossed as one ascends 
the hill to the eastward, their existence being proved by the numerous 
shafts which dot this point of the hill. A small body of quartz-porphyry 
is found on the slope of the hill toward South Evans gulch, between the 
Parting Quartzite and White Limestone, which may correspond to that 
found on the other side of the anticline in K-14. The contact of the Blue 
Limestone with White Porphyry has been proved in the Virginius, Tender- 
foot, and Cleveland shafts, where it is more or less replaced by vein mate- 
rial, and in the first is said to have contained large bodies of low grade and 
some rich ore. Southward across South Evans gulch this contact is practi- 
cally unprospected. 

The north slope of the anticline is proved on the north side of Evans 
gulch, in the United States Mint shaft (G-38), which is sunk in the shaly 
beds at the top of the Lower Quartzite. The northern rim of the anticline 
is buried below 400 feet of gravel of the Evans moraine, and it is only on 
the steeper slopes of Prospect Mountain, adjoining Little Evans gulch, that 
rock in place is found. Here the workings of La Harpe, Stillwell, Little 
Louise, Golden Eagle, and other claims have proved the contact of the main 
Gray Porphyry sheet and the overlying Weber Shales. The main Gray 
Porphyry sheet is not found east of the South Evans anticline, and there 
fore must thin out rapidly beyond these claims. The body of Mount Zion 
Porphyry, which crosses Evans gulch above the Ball Mountain fault, as 
already described (if, as supposed, an interbedded sheet), comes between the 
Weber Shales and the Weber Grits, commencing opposite where the Gray 
Porphyry sheet dies out. 


M'ke fault. The Mike fault runs more nearly parallel with the Weston 
fault than the Ball Mountain fault. On the south it extends a short distance 
beyond the limits of the map, as shown in discrepancy of outcrops on the 
south slope of Long and Derry TTill, but it cannot be traced beyond the 


alluvial deposits of Empire gulch, and apparently passes into an anticlinal 
fold on the west slope of Empire Hill. 1 On Long and Deny Ridge it is 
defined at the Kenosha tunnel, where a body of greenish quartz-porphyry 
on the east comes into juxtaposition with White Porphyry on the west; 
the former body belongs below the horizon of the White Limestone, the 
latter above that of the Blue. It descends into Iowa gulch near the point 
where the Long and Deny grade reaches the crest of the hill, crossing the 
gulch just west of the Now-or-Never (M-49) shaft; passes the western 
foot of the Printer Boy Hill to California gulch between its junctions with 
Eureka and White's gulches, and along the western slope of Breece Hill, 
through the workings of the Mike and Star mine, from which it receives its. 

In the latter region the surface indications do not prove its existence,, 
since White Porphyry is found on both sides; but its movement is proved 
by the relative depths of the Blue Limestone horizon under the White Por- 
phyry in the two shafts of the Park mine (0-1 and O-4). Its movement 
cannot be traced north of Adelaide park, and it is supposed to end at its. 
junction with the Breece Iron fault. 

Its movement of displacement at the south end is an upthrow to the 
east, which is about one thousand feet on Long and Deny Ridge and 
decreases to the northward. North of its junction with Pilot fault, in Iowa 
gulch, as above stated, the only data are derived from the Park mine, fromi 
which it is inferred that the movement is reversed and is an upthrow on the; 
west, gradually increasing from that point to a maximum of about three 
hundred feet. 

Pilot fault. Pilot fault might in one sense be considered the normal con- 
tinuation of the main Mike fault, from the fact that its upthrow is the same 
and that the amount of its displacement decreases continuously to the north 
until its movement is entirely lost in the body of Pyritiferous Porphyry on 
Breece Hill. Its direction diverges at first very sensibly from that of the 
Mike fault, being a northeasterly direction across the western slope of 

1 On the Mosquito map (Atlas Sheet VII), by an error of the engraver, which had been over- 
looked, it has been continued eonth of Empire gulch to a junction with Union fault on a line which is 
simply the boundary between White Limestone (c) and Lower Quartzito (6). 


Printer Boy Hill and bending to the north beyond the Pilot tunnel, from 
which it receives its name. It crosses California gulch a little above the 
Lower Printer Boy mine, and White's Hill just east of the shaft L-34. 
Beyond that point its position can no longer be defined, but it seems prob 
able that it passes through a slight anticlinal fold under the Breece Hill 
body of Pyritiferous Porphyry, the continuation of a fold farther north 
which is proved by the explorations of the shafts in the neighborhood of the 
Great Hope mine, above Evansville. 

union fault. The Union fault, which is principally developed south of 
the limits of the map, lias a direction nearly parallel with the Mosquito 
fault. As described in the preceding chapter, it crosses the western slope of 
Empire Hill and disappears to the southward in the granite area adjoining 
lower Weston gulch. Its displacement is an upthrow to the east, which 
from a null point at its south end increases towards the north, reaching a 
maximum of about one thousand feet at its junction with the Weston fault, 
in Iowa gulch below the Ella Beeler tunnel, where the Archean comes in 
contact with the upper portion of the White Porphyry above the Blue Lime- 

In the bed of Empire gulch Archean is exposed east of this fault, and 
the overlying Cambrian and Silurian beds can be traced, sweeping up the 
slopes on either side of the gulch, where not covered by the Empire 
moraines. A few shafts and tunnels have penetrated the moraine material 
to the underlying quartzite and silicious limestone. Such is the Little 
Annie tunnel, just east of the fault, on the south slope of Long and Deny 
Hill, which ran through moraine material into White Porphyry, and at 
whose end a winze was sunk into the underlying limestone. Farther east 
the Coffee, Louis Tell, California Rose, and Caledonian tunnels are run 
upon the contact of White Porphyry and Silurian limestone, which beyond 
them abut against the granite on the other side of Weston fault. 

Long and Derry Ridge. The structure of this ridge is best explained by 
Section I, Atlas Sheet XX. The actual line of the Union fault on the crest 
of the ridge is undefinable, since White Porphyry is found on either side. 
Two small outcrops are found near the crest of the hill, adjoining the Weston 
fault, which represent portions of the Blue Limestone above the lower beds 


of White Porphyry that have escaped erosion. Of these the more north- 
erly one is entirely replaced by iron-stained chert, which forms a rocky 
outcrop near the crest of the ridge 

On the north slope of the ridge, in the sharp angle formed by the two 
faults, the Ella Beeler tunnel ran in on granite and struck a coarse quartzite 
dipping southwest, which forms the base of the Lower Quartzite. 

From Union fault down to the Long and Derry mines the ridge is 
covered by the upper main body of White Porphyry, in which are included 
two comparatively thin beds of sandstone and shale belonging to the Weber 
Shale formation, associated with which are two small bodies of later quartz- 
porphyry. The dip of these beds is at a low angle to the eastward, the 
upper being shown by actual outcrops; the lower, which consists of shales 
with some sandstone, is shown in the Pride of the West (E-15), Camp- 
bell (E -25 and E-26), Gildersleeve (E-27), and Hoosier shafts on the 
south slope, and by the Herculaneum (E-14) tunnel on the north slope 
of the hill. 

The Blue Limestone is proved on the south slope of the hill by the 
workings of the Aerial Queen (E-34), Homestake (E-36), and other 
shafts, which have reached it through the overlying White Porphyry, and 
by the Himalaya (E-35) tunnel ; and on the north slope of the hill by the 
workings of the Long and Derry group of mines. 

On the steep face of the ridge facing Iowa gulch, above Long and Deny 
grade, is found one of the few distinct outcrops of the two bodies of lime- 
stone, the Blue and the White. Here they dip regularly to the eastward 
at an angle of 10 to 15, and are underlaid by a body of Green Por- 
phyry. The Belcher 1 tunnel (M-5) runs in on an ore body in the lower 
part of the Blue Limestone. Above this and a little to the eastward is a 
prominent black rock-mass resembling at a little distance the outcrop of a 
body of iron ore. This is the upper portion of the Blue Limestone body, 
which is here largely replaced by chert and oxides of iron and manganese. 
Immediately above this and directly under the porphyry is a body of con- 
glomerate, from 25 to 30 feet thick, which is assumed to be a portion of the 
Weber Shales cut off from the main body by the porphyry sheet. 

1 Wrongly given lu the Index of Shafts, etc., as Beecher. 


Long and Derry mines. The Long and Deny workings consist of a number 
of shafts and tunnels, the former of which the Dana (M-3) and Por- 
phyry.(E-37) reach the contact through the overlying White Porphyry. 
Two tunnels on the Faint Hope claim (M-2 and M-4) start in the lime- 
stone, the latter reaching the contact between porphyry and limestone at 
183 feet, while the Long and Derry tunnel (E-32) is run in through the 
porphyry for a distance of 400 feet. Mineral action has here extended 
down into the limestone body, and ore is found not only at the contact, but 
in irregular chambers, at considerable depths below it. 

Dikes. Immediately in front of Faint Hope tunnel (M-4) is an out- 
crop of Gray Porphyry almost identical lithologically with the main sheet 
of Gray Porphyry. This is part of a vertical dike fifty to sixty feet wide, 
which can be traced past the Belcher Mine, across Iowa gulch, to the Minor 
tunnel on Printer Boy Hill. There are three of these vertical dikes, which 
can be most distinctly seen on the steep slopes of Printer Boy Hill, where 
in some cases they stand as projecting outcrops, the adjoining rock having 
been eroded away. They vary from thirty to fifty feet in thickness, and, 
as well as could be traced on the surface, are nearly parallel and all of the 
same character of rock. 

For some distance from the Long and Derry mines westward no actual 
rock outcrops are found on the surface of the ridge. Along its southern 
slope the presence of the White Limestone and of an included body of 
coarse-grained quartz-porphyry, somewhat resembling the Josephine Por- 
phyry, was detected by means of several small prospect holes, too unim- 
portant to have been indicated on the map. The fine-grained Green Por- 
phyry is much decomposed and of lighter color than in Iowa gulch. The 
secondary ridge or shoulder of the main ridge, at the very edge of the map, 
overlooking Empire gulch, is formed by the Empire north moraine, through 
which few, if any, prospectors have succeeded in reaching the underlying 
rock. On the north slope of the ridge, as already mentioned, the White 
Limestone forms continuous outcrops, crossing which can be detected the 
vertical dikes which cut the strata at right angles. 

Iowa guich. In the bed of Iowa gulch, as shown in outcrops along the 
creek at the foot of the Long and Derry grade, and in the Minnehaha (M- 1 5 


and M-16) and other shafts, is a considerable body of compact Green 
Porphyry, apparently part of an interbedded sheet underlying the White 
Limestone. This extends some distance above the bridge, but opposite the 
Belcher tunnel the outcrops of gray limestone belonging to the Silurian 
formation are found resting on it, dipping 12 to the northeast. In this 
limestone can be seen the outcrop of a body of coarse-grained quartz-por- 
phyry similar to the rock of the dikes. For several feet from its contact 
the limestone seems to be hardened and silicified, with small veins of por- 
phyry running into it. It is probably either an offshoot from one of the 
dikes already mentioned or a separate dike. 

From the steep cliff of White Porphyry above the Long and Deny 
tunnel an immense talus cone of tabular and sherdy fragments of White 
Porphyry spreads out into the valley, so as almost to block it up and to 
completely cover the outcrops of the Blue Limestone. At first glance it 
would seem that this immense accumulation of de"bris must be due to some 
unusual cause. As none such could be found, the great height and steep 
slope of the ridge which is occupied by the body of White Porphyry and 
the peculiar weathering of this particular rock, which, under the influence 
of atmospheric agents, disintegrates very rapidly into tabular sherdy frag- 
ments, must be considered an adequate explanation. These fragments, 
which are very light in proportion to their superficial area, slip down rap- 
idly under the influence of rain, snow, and frost, and soon accumulate in 
very considerable talus cones at the foot of any steep slope whose surface is 
largely composed of White Porphyry. Owing to the depth to which the 
rock surface is buried under this debris, the contact has not been prospected 
between the Long and Deny mines and the First National. 

Printer Boy Km. On the north side of Iowa gulch, along the south slope 
of Printer Boy Hill, the contact of the Blue Limestone and White Porphyry 
is well marked by a series of tunnels and shafts. The First National shaft 
finds ore at the contact after sinking through 75 feet of White Porphyry. 
The Seek-no -further (E-38), Mammoth (E-39), and some other shafts 
have also reached the contact through the porphyry. The Minor tunnel 
and the upper tunnel of the Florence are at the highest part of the con- 
tact, while the J. D. Ward shaft, on the summit of the hill, sinks 300 feet 


through porphyry to reach it. From the Florence westward the contact 
slopes down again towards Iowa gulch through the Sangamon (M-24) tun- 
nel, Brian Boru, Wilson (M-38), Blacktail (M-40), G. M. Favorite (M-44), 
and other claims and crosses the gulch. 

On the north bank of the creek southeast of the G. M. Favorite are 
found outcrops of Parting Quartzite, consisting here of sandy beds with some 
purplish shale, and of the top of the White Limestone, dipping 20 to the 
westward. These facts and the outline of the Blue Limestone on the north 
side of Printer Boy Hill and alongCalifornia gulch show that under this hill is 
an anticlinal fold whose axis runs north and south through the west end of the 
hill, and which, like the other folds, has a steeper slope to the west. On Long 
and Derry Ridge its western half has been cut off by the Mike fault. The 
White Porphyry above the Blue Limestone, on the western side of this fold, 
is cut in the Now-or-Never (M-49) and other shafts in Iowa gulch; and 
the Nestor (M-'28) shaft, on the crest of the ridge, has reached limestone 
after passing through 1 70 feet of White Porphyry. 

On the north side of Printer Boy Hill, along the upper portion of Cal- 
ifornia gulch, the presence of the Blue Limestone is proved in the Lovejoy 
shaft (M-29) and in the workings of the Eclipse mine (M-7 and M-9). 
The Lovejoy passes through the limestone into an underlying bed of 
quartz-porphyry, which is also found in the Stars and Stripes tunnel and 
which very closely resembles the Green Porphyry body found in Iowa 
gulch under the White Limestone. The Eclipse tunnel (M-7) runs in 
170 feet through limestone and then strikes the contact of the overlying 
White Porphyry, which it follows. The porphyry is here unconformable 
to the limestone, cutting across its stratification. The dip of this limestone 
is 15 to the south. The lower tunnel (M-9), about thirty feet below this, 
is run on the contact of Blue Limestone and Parting Quartzite; both con- 
tacts show vein material. The small thickness of limestone included be- 
tween Parting Quartzite and overlying White Porphyry is an additional 
evidence that the latter is here cutting across the Blue Limestone. 


Head of California gulch. At the head of California gulch, on the north, 
the Ohio Bonanza tunnel (not on map) runs in at the surface of a fragment 
of Blue Limestone which has been left above the White Porphyry; still 
higher up, the Snowbird shows a sandstone completely impregnated with 
pyrite and surrounded by Pyritiferous Porphyry, which is supposed to be 
a detached fragment of Weber Shales. On the south side the Tinker 
(E 43) shaft has penetrated the White Porphyry to the underlying lime- 
stone. The Belle Vernon shaft was sunk through 80 feet of Wash and 150 
feet of White Porphyry without reaching it. 

The occurrence of Wash here in California gulch is significant, as 
showing that the Iowa gulch glacier must at one time have filled the 
valley to the height of the saddle east of Printer Boy Hill and a part of its 
moraine material must have been pushed over into the head of California 
gulch, or else that a portion of the glacier actually extended over the ridge. 
In the lower part of the gulch there is no evidence of glacial action. 

Pyritiferous Porphyry. On the south side of Green Mountain, overlooking 
Iowa gulch, is the Alta tunnel, which runs 30 feet through Wash, 63 feet 
through White Porphyry, and 192 feet into the overlying body of Pyri- 
tiferous Porphyry, here dipping northeastward, while the North Star shaft 
(E 23), just above it, is sunk in Pyritiferous Porphyry. The rock here, 
though characteristic, does not contain much pyrite, except at the end of 
the Alta tunnel, where it is associated witli stains of galena. Little SchuyL 
kill shaft, in the south head of California gulch, has been sunk through 
Pyritiferous Porphyry into the underlying White Porphyry, while the 
Ella and adjoining (F-38) tunnels are run in Pyritiferous Porphyry. All 
these shafts are just below the Weston fault, and the Pyritiferous Porphyry 
belongs to the main sheet which covers the greater part of the slopes of 
Breece Hill, and which corresponds with the lowest sheet east of the fault, 
viz, that found just above Idaho Park. 

In California gulch, as already stated, there is a still lower body of 
Pyritiferous Porphyry, whose rock, though not absolutely identical with 
the other, resembles it closely enough to form a part of the same body, 
and which comes at different points in contact now with the Blue Limestone 
and now with the White Limestone. It is therefore supposed to be cutting 


up across these beds. It is proved on the south side of the gulch in the 
Ben Franklin tunnel and shaft and in the Kid, Burt (M-13), Soda Card 
(M-20), and other shafts. The Wynan shaft (M-12) has sunk into it 
through the Parting Quartzite, while the Eclipse No. 2 shaft (M-8) is still 
in White Limestone. 

North side of California gulch. On the north side of Upper California gulch 
the Parting Quartzite outcrops at Pigtail gulch and can be traced in a 
number of prospect holes and in the slide. In the Iron Duke it shows iron 
ore which deflects the needle. The Frank shaft (L-26) has gone through 
the White Porphyry into underlying White Limestone. The Charlie P. 
(L-28) tunnel in Pigtail gulch and the P. I. R. (L-29) and Comstock 
tunnels run also in White Porphyry, which gradually thins out to the 
westward between the two bodies of Pyritiferous Porphyry. The Comstock 
runs in on the contact of this White Porphyry and a thin layer of dark, 
impure limestone, which dips 15 to the northeast and is considerably min- 
eralized. From it has been obtained serpentine similar to that found in the 
Red amphitheater of Buckskin gulch. The lithological character of this 
limestone gives no definite indication of its horizon. The presence of the 
serpentine allies it to the White Limestone, but general stratigraphical con r 
siderations favor its reference to the horizon of the Blue Limestone. 1 In 
either case it is evident that the White Porphyry, as well as the lower body 
of Pyritiferous Porphyry, is here cutting up across the strata. 

Primer Boy Porphyry. Lower down the gulch the portion of Printer Boy 
Hill included between the Pilot and Mike faults, a wedge-shaped block of 
ground which seems to have been let down between them, shows at the 
surface only a body of quartz-porphyry, which is noticeable as being that 
in which the principal developments of gold ore have thus far been found, 
those of the Printer Boy and "5-20" mines. This porphyry is generally 
decomposed and does not correspond exactly to any other found in the re- 
gion, though somewhat resembling the Gray Porphyry. It has a greenish- 
gray matrix, owing its color doubtless to the decomposition of bisilicates, with 
large white opaque feldspars often two to three inches long. Its eastern 
limits are defined by the Abe Lincoln (M-36), Nightingale (M-33), and 

'Oil the map this limestone is outlined, hut the blue color has been omitted. 


Pilot tunnels, the latter of which is directly on the fault line. The workings 
of the various shafts of the Printer Boy mine follow a crack or fissure in 
the body of this porphyry and cut an apparently included body of Pyri- 
tiferous Porphyry. The Gray Eagle tunnel in Eureka gulch is on the 
western limits of the body, in contact with an underlying mass of Pyritif- 
erous Porphyry. The Fitz-James (?) shaft (M 54), at the head of Eureka 
gulch and just west of the Mike fault, after penetrating the Wash, was sunk 
through a large mass of decomposed porphyry, apparently of two kinds, 
one supposed to be Pyritiferous Porphyry, and reached the White Porphyry, 
still below that. This body of Pyritiferous Porphyry is apparently part of 
the main sheet that covers Breece Hill, and seems to thin out to the south 
and west. It forms the bed of California gulch from Oro City up to the 
Pilot fault, while the underlying White Porphyry outcrops below Oro City. 
The shaft L 44, still on the east side of the Mike fault, is sunk in this same 
underlying White Porphyry. 

Mike mine. The Mike mine, just east of the head of Nugget gulch, is 
. also sunk in the White Porphyry, a little west of the line of the fault. The 
porphyry here shows a very peculiar semi-columnar structure, which is evi- 
dently due to the pressure and movement caused by the fault. It separates 
out in long, flattened prisms, and the porphyritic structure of the material, 
which is reduced practically to a clay, is almost lost. The flat surfaces of the 
prisms are parallel to the fault plane, and not at right angles to it, as would 
be the case if it were the columnar structure of a dike. 1 

Breece Hill. The whole surface of Breece Hill north of California gulch 
and east of the Mike fault shows nothing but Pyritiferous Porphyry. In 
the weathered 1'ock, as has already been stated, pyrites are not generally 
found, having been dissolved out by surface waters; but wherever it is ex- 
posed bj shafts or tunnels it is found to contain, at a distance from the sur- 
face, a most remarkable quantity of fine crystals, varying from almost mi- 
croscopic size tc one-eighth of an inch or more in diameter. These are fre- 
quently concentrated along natural joints in the rock, and in such cases 

'Developments made iu this mine since the completion of field- work have confirmed the asser- 
tions made in regard to tho structure at this point, and shown on Atlas Sheet XVIII, Section FF. The 
contact was struck in the shaft at a, depth of 426 feet, and the fault proved by a drift run east. The 
formation dips 20 to tho southwest., showing that the amount of basiuing-up was under rather than 
over valued. The ore found is principally snlphurets and eaid to be exceptionally rich. 


sometimes accompanied by a slight deposit of galena, as, for instance, in the 
Printer Girl, Lalla Rookh, and Lillie tunnel. Thus far no valuable deposits 
of ore have been found in this body, nor, except on its northern edge, has 
its thickness been determined. On its southern and western borders it is 
found to be underlaid by White Porphyry, and on the northeastern edge 
the main sheet of Gray Porphyry intervenes between the two. As already 
explained, it is evidently cutting up across the formations in California 
gulch, and on White's Hill it rests directly on the lower sheet of White Por- 
phyry, probably cutting up across Blue Limestone and upper White Por- 
phyry to the north, as shown in the north and south sections, K and L, 
Atlas Sheet XXI. The numerous prospect shafts which have been sunk 
in this body were mostly deserted at the time of this visit, so that definite 
data as to their depth could not always be obtained The Comstock (L-17) 
and Tribune (L-ll) shafts had reached a depth of 300 feet and were 
still in it. The Cumberland shaft, at a depth of 450 feet, had struck the 
underlying Gray Porphyry, into which it had penetrated 25 feet. The 
Lady Jane shaft, a little to the west, had also reached the Gray Porphyry, 
but its depth was not ascertained. At the northwestern corner of the body, 
the Ishpeming shaft (L 42) and the Kent shaft (L-43) had also pene- 
trated the Pyritiferous Porphyry, the former to a depth of 90, the latter of 
100 feet, and reached the underlying White Porphyry, showing that the 
Pyritiferous Porphyry rapidly thins out in this direction. 

Breece fault. The northern limits of the body are sharply defined by 
the Breece cross-fault. This fault, which has porphyry on either side and 
at its western end identically the same rock, cannot be traced on the sur- 
face. It has a nearly east and west direction, extending across Adelaide 
Park, through the Silver Cloud (K-59) and Eureka shafts, north of the 
Kent, south of the Breece Iron, north again of the Glasgow and Comstock 
shafts, which are in the Pyritiferous Porphyry, and south of the Pennsyl- 
vania shaft (K-19), which is in the Gray Porphyry The porphyry in the 
Silver Cloud shaft shows the same evidence of pressure as that already de- 
scribed in the Mike, while the Eureka shaft shows a breccia material made 
up of small fragments of what would appear, under the microscope, to be 
volcanic rock of the rhyolitic type. No satisfactory explanation of this 


peculiar occurrence has been found, nor can it be hoped for until work on 
the now abandoned shafts shall be resumed. 

The upthrow of the Breece fault is to the north, and apparently reaches 
a maximum at its -eastern end, where it is estimated at about 500 feet. 

Breece iron mine. The Breece Iron mine, which is situated on the western 
slope of Breece Hill, overlooking Adelaide Park, has a remarkable deposit 
of red hematite, mixed with magnetite, which occurs at the contact of the 
main sheets of White and Gray Porphyries. Its ore is found at the surface 
in two bodies, having a maximum thickness of nearly thirty feet each, the 
lower of which is underlaid by White Porphyry, while, between it and the 
upper body, which is apparently an offshoot from the main body, is a sheet 
of decomposed porphyry which has certain resemblances both to the Pyri- 
tiferous and to the Gray Porphyry. This deposit is apparently due to the 
oxidation of a mass of iron pyrites, which were brought to their present po- 
sition in solution in a similar manner to the other ore deposits of the region. 
Indications of iron are found along the contact line between the White and 
Gray Porphyries, to the eastward, but as yet no considerable bodies of iron 
have been developed. 

West of the Breece mine, the Superior and Mountain Boy, on the ridge 
connecting Breece and Yankee Hills, have also struck a considerable body 
of iron between the Gray and White Porphyries, dipping north. This may 
be a continuation of the Breece Iron body, the intermediate portion having 
been removed, by the erosion of the head of Stray Horse gulch, which has 
brought to the surface the White Porphvry underlying the Gray. On the 
other hand, while the Breece iron is an anhydrous red hematite, the mate- 
rial developed in these shafts consists of brown hematite and bluish-gray 
chert, the usual replacement, material of Blue Limestone, for which reason 
the outcrop is indicated on the map by the color of that formation. The 
Theresa (K-57)shaft, to the northeast, finds shales impregnated with pyrites 
at the contact of the two porphyries, at a depth of 325 feet. 


The line of Mike fault, if continued northward, would pass through an 
anticlinal fold, whose crest reaches from the north slope of Yankee Hill to 


the southwest foot of Canterbury Hill, just below the forks of Little Evans 
gulch. To this line converges also the northern end of the Iron fault, whose 
throw becomes null at the crest of the fold. The simplest expression of the 
structure of the region between Fryer Hill and Weston fault north of 
Stray Horse gulch is that of a synclinal basin in Little Stray Horse Park, 
the eroded crest of an anticline at Yankee Hill, and a syncline farther 
eastward, whose rim is partially cut off by the Weston fault. The intrusive 
masses of porphyry here associated with the regular sedimentary series are 
a lower sheet of White Porphyry between the White Limestone and Part- 
ing Quartzite, an upper sheet of White Porphyry above the Blue Limestone, 
and the main sheet of Gray Porphyry above it. This comparatively simple 
structure, resulting from folding alone, which obtains along the line of Big 
Evans gulch, on the north slope of Yankee Hill, is complicated on the south, 
first, by the displacement of the Iron fault, which cuts diagonally into the 
crest of the fold after crossing Stray Horse gulch west of the Argentine 
tunnel and passing between the Double- Decker and Highland Mary shafts, 
the east and west shafts of the Hard Cash mine, and through the eastern end 
of the Chieftain tunnel ; secondly, by the movement of the Adelaide cross- 
fault, which extends from the Iron fault opposite the mouth of the Argentine 
tunnel, just west of the Laura Lynn shaft, to the saddle between Adelaide 
Park and the head of Nugget gulch ; and, thirdly, by the intrusion of sev- 
eral irregular masses of Gray Porphyry. 

synciine east of Yankee Hiii. The greater part of the surface between 
Yankee Hill on the west, the mouth of Lincoln gulch on the east, and the 
steep slopes of Prospect Mountain on the north is covered by the main 
sheet of Gray Porphyry, which directly overlies the upper sheet of White 
Porphyry. The White Porphyry only comes to the surface along the flanks 
of the Yankee Hill anticline and in the valley of Upper Stray Horse gulch 
and Adelaide Park. The contours of the map in the latter region would 
seem at first glance to negative the idea that the exposure of porphyry was 
simply due to a deeper erosion, since they show in the White Porphyry 
area not only a valley but also the summit of a ridge. It must be borne 
in mind, however, that these contours represent the actual surface of the 
ground and not the rock surface, whereas the geological outlines refer only 


to the latter ; and the records of the depth of Wash in various shafts here 
show that this ridge was formed by the south moraine of the Evans glacier. 

The Keystone (K-58), Uranus (K-53), Tiger bore-hole (K-47), 
White Check (K-48), Tootie Gaylord (K-46), Big Six, and the lower 
shaft of the Breece Iron have found White Porphyry immediately under 
the Wash, the latter shaft being sunk into it for a depth of 350 feet, while 
the Tiger bore-hole, at a depth of 500 feet, was, as well as could be ascer- 
tained, still in it. 

On the upper northwest slope of Breece Hill are a number of shafts 
in the Gray Porphyry, most of which have not gone through it, The 
Fenian Queen, adjoining the road, passed. through 150 feet, respectively, of 
Gray tind White Porphyry into the underlying Weber Shale. The Nora, 
near the foot of the slope below it, reached the contact under the Gray Por- 
phyry without finding any intervening White Porphyry. 

A group of shafts in the neighborhood of the Great Hope and Across- 
the-Ocean find the Blue Limestone at a comparatively small depth, in general 
not more than seventy to eighty feet, and the White Porphyry between it 
and the Gray Porphyry is either very thin (fifteen to twenty feet in the 
shafts above mentioned) or entirely wanting, as in the Bosco (K-28). 
The Great Hope, after passing through these sheets of White and Gray 
Porphyry, found 60 feet of vein material, and reached the Parting Quartzite, 
here carrying gold, at a depth of 1 30 feet. On the other hand, directly west 
of these~shafts, the Independent has been sunk 4"20 feet in the Gray Por- 
phyry and the H. M. L. 160 feet without reaching the bottom, while the 
Onota, which is 150 feet lower than the Independent, found vein material 
at a depth of 400 feet, after passing through 300 feet of Gray Porphyry and 
100 feet of White Porphyry. There is, therefore, evidently a synclinal basin 
between the Great Hope and the crest of Yankee Hill, and also some indi- 
cation that the contact sinks to the eastward before rising up under the 
influence of South Evans anticline against Weston fault ; in other words, 
that there is a slight ridge or secondary fold in the strata on the line through 
these shafts, as shown in Section D, Atlas Sheet XVIII. 

The Little Prince, on this same line, but higher up on the slope of Breece 
Hill, reached the Blue Limestone horizon, which is here represented by a 


mass of silicious vein material containing pockets of carbonate, at a depth 
of 230 feet. Inasmuch as this shaft starts at an elevation of about two hun- 
dred and fifty feet above the Great Hope, the absolute level of the contact 
is here even higher than at the Great Hope, as shown in Section L, Atlas 
Sheet XXI. 

A number of shafts near this the Galesburg (K 33), the White 
Prince (K-36), and Nettie Morgan (K-38) have also reached the 
contact after passing through Gray and White Porphyry. The Big Six, 
at a depth of 300 feet, and the Tiger bore-hole, at 500 feet, as already men- 
tioned, were still in White Porphyry, showing that in a southwest direction 
it thickens very rapidly. Between Evans gulch and Little Evaift the moraine 
ridge buries the rock surface to such a depth that except at its western end 
it has not been reached. 

On the slope of Prospect Mountain, as will be shown later, the Gray 
Porphyry is overlaid by the Weber Shales. The underlying White Por- 
phyry is thinning out to the northeast, while still farther north the Mount 
Zion Porphyry comes in between the Gray Porphyry and the Weber Shales. 

Yankee Hiii anticline. Across the west slope of Yankee Hill, just below 
its crest, runs the axis of an anticlinal fold, which in Evans gulch prob- 
ably bends to the southwest to connect with the anticline shown at the 
forks of Little .Evans, at the south base of Canterbury Hill. The rock 
surface in the crest of the fold on Yankee Hill is formed of White Por- 
phyry, belonging to the sheet which comes between the White and Blue 
Limestones, this region being northeast of the imaginary line already men- 
tioned as running southeast from Fryer Hill, along which the main sheet 
of White Porphyry splits in two, one portion remaining above the Blue 
Limestone and the other being found below it. 

North slope of Yankee HHi. The regular succession of beds on either side 
of the axis of this anticlinal fold is best shown in the shafts on the north 
side of the hill. In Johnson gulch the Andy Johnson (P 1) shaft reaches 
the contact after passing through both the main sheet of Gray Porphyry 
and the underlying White Porphyry, the latter being here 84 feet thick. 
The Bevis No. 3 (P-5), Bevis Discovery (P-6), and the Boulder Nest 
(P 8) shafts have started in White Porphyry and readied the contact 


at depths below rock surface of 170 feet, 45 feet, and 50 feet, respect- 
ively, the latter having also 70 feet of Wash. The Hidden Treasure tun- 
nel (P-7) is run in on the contact line. The William and Mary tunnel 
(P 12) runs on the contact of the Parting Quartzite and White Limestone, 
and the Sappho shaft develops the contact of White Limestone, dipping 10 
east, with underlying White Porphyry. This White Porphyry is the lower 
sheet which occurs normally between the Blue and White Limestones, and 
the White Limestone developed in the two shafts is evidently a portion 
split off from the main formation by this porphyry sheet and left above it. 
The White Rabbit (P-17) and Little Stella are sunk in the lower sheet 
of White Porphyry, the latter having reached the main body of White 
Limestone below it. The Bismark (P-20) and Holden (P-24) are sunk 
in the lower portion of the White Limestone, near the crest of the fold, the 
latter having reached the Lower Quartzite beneath it. 

On the west of the fold the J. B. Grant and the Dania (P 30) are 
sunk through the lower sheet of White Porphyry into the underlying Lime- 
stone, while the First Chance (P-37), Bobtail (P-40), and the Cordelia 
Edmonston find Blue Limestone, or the vein material which replaces it, 
immediately below the Wash. These outcrops form part of the eastern 
member of the Little Stray Horse syncline. 

south slope of Yankee Hiii. On the south side of Yankee Hill, towards Stray 
Horse gulch, the simple anticlinal structure shown above is considerably 
complicated. The first disturbing element is the Iron fault, which may be 
regarded as the result of an anticlinal fold, since the beds dip away from it 
on either side. Hence, an eastern dip is found here in a position on the 
slope corresponding to the western dip shown in the last-mentioned shafts, 
and, by the movement of the fault, Lower Quartzite and Archean outcrops 
are exposed directly east of it. Besides this, there extends from the crest 
of the hi)l southward across Adelaide Park a large mass of porphyry resem- 
bling Gray Porphyry, which splits the Blue Limestone, and which, from 
the meager data obtainable, seems to be cutting up across the formation 
from below. For convenience of description this mass of porphyry will 
be called the Adelaide body. Near the crest of Yankee Hill a considerable 
body of iron-vein material has been developed, which passes into Blue 



Limestone to the south and belongs to the eastern member of the Yankee 
Hill anticline, being a continuation of that found in the Andy Johnson 
and other mines. 

The Little Champion (P-ll) and Greenwood shafts were still in this 
body of vein material, the former at a depth of 200 feet, after having passed 
through 30 feet of Wash and 15 feet of White Porphyry. The Clara Dell 
shaft, close by, found Wash, 126 feet; vein material, 5 feet; White Porphyry, 
95 feet; Adelaide Porphyry, 20 feet; and White Porphyry again, 121 feet. 
The Rothschild (P-9) was sunk 65 feet in Adelaide Porphyry, while the 
Leavenworth (P-4), a short distance east, reached the Blue Limestone after 
passing through 220 feet of White Porphyry without finding the Adelaide 
body, which must therefore go down very steeply on this line. The Louis- 
ville (O-13) on the north and the Laura Lynn (O-15) on the south side 
of Adelaide park are both in Adelaide Porphyry, while the Day bore-hole 
(O 14) in the middle furnishes the following important section, as derived 
from an examination of drill-cores: Adelaide Porphyry, 100 feet; White 
Limestone, 87 feet; Adelaide Porphyry, 39 feet; White Limestone, 37 feet; 
Lower Quartzite, 116 feet; Archean, 2 feet. It thus appears that the Ade- 
laide Porphyry is here in part immediately above the White Limestone, 
whereas in the Clara Dell it was in the lower body of White Porphyry, which 
is wanting at this point. From the extremely short distance between the 
Rothschild and Leavenworth and from the great depth of the Blue Lime- 
stone in the latter, it is assumed that a probable angle of dip of the Blue 
Limestone would bring it to the surface near the former, were it not that it is 
here cut off by the Adelaide Porphyry, which must cross it nearly vertically. 
South and east of the Day bore-hole again, the Park (O-4) shaft, the shaft 
0-6, and the Lily (0-5) shaft find Blue Limestone beneath the Wash, 
the last having reached White Porphyry beneath it. In the two latter 
shafts and in the eastern Park (O-l) shaft the limestone has a cream- 
colored tint, resembling decomposed White Porphyry, while in the west- 
ern Park shaft it has the characteristic blue-gray color. The underlying 
White Porphyry is cut in the adjoining shafts (0-10), (O-12), and Keno 
(0-11), while Keno (O-7)- is near the probable line of the Adelaide 
fault. The Horseshoe shaft, just south of these, at the head of Nugget 


gulch, passed through over four hundred and eighty feet of the upper 
sheet of White Porphyry before reaching Blue Limestone. 

Adelaide fault. The Adelaide cross-fault follows nearly the bed of Stray 
Horse gulch from the Iron fault up as far as the Adelaide smelter, from 
which point it bends southward, passing to the south of the Laura Lynn 
shaft. In this portion, however, it is impossible to determine its location 
with any approach to accuracy, as but few shafts are sunk and at its east- 
ern end White Porphyry occurs on either side of it. Its displacement is 
slight, its upthrow being on the northeast, and probably reaching a maxi- 
mum at its eastern end, It might be considered as a branch of the Mike 
fault, that fault having split into two at its northern extremity. 

It must be admitted that the triangular piece of ground in Adelaide 
Park between the Mike fault and the Adelaide cross-fault, in which the few 
deep shafts that have been sunk are mainly in different varieties of porphyry 
which the miners do not distinguish apart, shows a complication of struct- 
ure of which the explanation afforded by the map and sections may not 
prove entirely accurate when more extended explorations are made. There 
seems little doubt, however, that the irregular body of Adelaide or Gray 
Porphyry has been forced up directly from below somewhere in this re- 
gion; that it crosses the strata, and by thus interrupting the currents has 
been influential in determining the deposition of metallic minerals in this 
neighborhood, which are not only very abundant, but very irregularly dis- 

southwest slope. On the southwest slope of Yankee Hill the succession 
of outcrops indicated by the shafts is as follows: The Shenango (P-16) 
and Logan No. 2 shafts are in White Porphyry, below the Wash, while the 
Woodruff and Red-Headed Mary (P-22) have penetrated this body and 
reached the White Limestone beneath it. The shaft P-25 finds White 
Limestone below the Wash; the Hard Cash (P-31) and the Moonstone 
(P-32) shafts are in Lower Quartzite, and the Hard Cash (P-35), Logan 
No. 1 (P-27), and Silver Basin shafts have penetrated the Lower Quartz- 
ite to the underlying Archean. The Double-Decker shafts (P-47 and 
P-48) have been working on a body of gold ore in the Lower Quartzite, 
near the junction of the Adelaide and Iron faults. 


Moraines. The depth of Wash, where it could be obtained, affords data 
for locating the limits of the south branch of the Evans glacier. These 
nearly coincide with the bed of Stray Horse gulch, which has been eroded 
along the contact of its moraine with the rock surface to the south. South 
of this line there is practically no Wash, while the line of shafts just north 
of it show the following depths of moraine material: Leavenworth (P-4), 
207 feet; Rothschild (P-9), 260 feet; Clara Dell, 126 feet; Woodruff, 148 
feet; Logan, 100 feet; Silver Basin (P-33), 231 feet; Indiana (P-53), 180 
feet; Raven, 200 feet; Right Angle (P-69), 200 feet; Hunkidori (in the 
gulch), 35 feet; Denver City shafts, 180 feet. 


The area west of the Mike fault is divided into three faulted blocks by 
the displacement of the Iron-Dome and Carbonate faults. North of the 
line of Stray Horse gulch these faults merge into folds, and the structure is 
that of a series of anticlines and synch'nes, of which the Yankee Hill anti- 
cline and syncline have just been described. In what follows, the areas 
included between the two faults will be first taken up; then the Little Stray 
Horse Park syncline and Fryer Hill double anticline ; after that the Pros- 
pect Mountain region north of Evans gulch, in which the folds are merged 
into one broad anticlinal and synclinal fold, and finally the "as yet unknown 
mesa region under Leadville itself. 

iron-Dome fault. The Iron fault has been actually cut by underground 
workings and its plane explored to a greater extent than any other in the 
region, so that the line of its intersection with the rock surface is the most 
accurately determined, and perhaps for this very reason the most irregular. 
This irregularity has no doubt been exaggerated by the effect of erosion, 
and if the intersection of the fault plane with the rock surface were in a 
horizontal plane it would show less abrupt curves, but still present a marked 
contrast to the lines usually employed to represent fault outcrops. 

At its north end, on the west slope of Yankee Hill, as already shown, 
it merges into an anticlinal fold. Its plane is first cut at the end of the 
Chieftain (P-43) tunnel, which runs 360 feet in an average direction S. 
55 E. through Blue Limestone and vein material, much compressed and 


broken, and passes suddenly into granite ; the plane of the fault is here 
nearly vertical. South of this point it passes between the (P-46) shaft 
of the Hard Cash mine in vein material on the west and the two (P-31 
and P-35) which are in Lower Quartzite on the east. It crosses Stray 
Horse gulch between the Argentine tunnel and the Devlin shaft, then is lost 
sight of in an area of White Porphyry, in which it bends to the west, and 
is next seen in the Codfish Balls (0-37). Its course beyond this through 
the mines of Iron Hill will be described in detail in Part II, Chapter II. 

Beyond California gulch it is again lost sight of in porphyry, but its 
line would carry it into the axis of a synclinal fold between California and 
Iowa gulches. The actually proved continuation of its movement is along 
the California fault up California gulch to the Dome fault, which runs south 
across Dome Hill and in Iowa gulch passes into a probable anticlinal fold. 
The displacement of this fault is an upthrow on the east, its maximum of 
about one thousand feet being reached opposite the Iron mine, and decreas- 
ing both to the north and south. 

The area between Mike and Iron-Dome faults from the southern bound- 
ary of the map to the Adelaide cross-fault is practically a block of easterly- 
dipping beds, the surface being principally formed by the main sheet of 
White Porphyry, with a fringing outcrop on the west of the Blue Lime- 
stone, and, where erosion has cut deep enough before the Iron Dome fault is 
reached, by those of the lower formations. These are actually exposed 
only on the south slope of Iron Hill, facing California gulch. 

Long and Derry Ridge. On Long and Deny Ridge, west of the Mike fault, 
the underlying rocks are buried beneath the moraines of Empire and Iowa 
gulches, and, as shown on the general map, by Lake beds, so that the indi- 
cations afforded by shafts of the position of the outcrops of Blue Limestone 
are compai-atively rare. As far as known, the Echo and Hoodoo, at the head 
of Thompson gulch, are the only ones that have proved it, the one at a 
depth of 160 feet, the other at a depth of about one hundred and ten feet. 

Josephine Porphyry. The Josephine, Pine Tree, Aurora, and other shafts 
have developed a body of porphyry which has been called after the first- 
named shaft, in which it has been best developed. It apparently forms a 
sheet between the White Porphyry and underlying Blue Limestone, the 


Pine Tree having reached it after crossing 145 feet of White Porphyry. It 
is a coarse-grained, gray rock, containing white and rather glassy feldspars, 
quartz in smoky, rounded grains, and biotite in distinct crystals. Cavities 
filled with white opaque calcite are frequently found. The gray color of 
the groundmass is due to numerous black specks, many of which are ore 
grains and others minute biotites. The feldspars under the microscope are 
seen to be partly triclinic, although nionoclinic feldspar predominates. Both 
quartz and feldspars contain inclusions of the groundmass and glass inclu- 
sions. In the quartz, in one case, fluid inclusions with a moving bubble 
are also observed. Calcite is present in considerable quantity, both in the 
groundmass and in the feldspars. In general, from the microscopical exam- 
ination alone, Mr. Cross would have been inclined to class this rock among 
the Tertiary eruptive rocks. If it be so, it is probably not an intrusive sheet, 
as has been assumed, but an irregular dike. These indications do not, 
however, seem sufficiently decisive to outweigh those of its field habit and 
mode of occurrence, which ally it to the later intrusions of porphyry of pre- 
Tertiary age. 

Lake beds. Lake beds were found in a prospect hole near the shaft 
M-41, were passed through by the Pine Tree shaft, and penetrated to a 
depth of 175 feet in the Continental shaft (M-50), which was sunk in the 
Iowa south moraine. Several shafts and tunnels have been run in this mo- 
raine and have very probably penetrated the underlying Lake beds, but, as 
far as known, have not reached rock in place on the south of Iowa gulch. 

Iowa gulch. On the north bank a number of shafts and tunnels have 
proved the existence of outcrops of Blue Limestone in the vicinity of the Nisi 
Prius workings, one of whose tunnels has followed the contact for a distance 
of 700 feet, disclosing a considerable body of contact vein material. The 
Little Birdie (N-18) tunnel was driven 200 feet in the moraine without 
reaching rock in place. 

Dome Ridge. On Dome Ridge the principal developments have been 
made near the outcrops of the Blue Limestone, the few shafts in porphyry 
at considerable distance east of this not having been sunk to any great 
depth. No definite data are therefore obtainable as to the aggregate thick- 
ness of the White Porphyry sheet. The principal workings are those of the 


Dome, Rock, and La Plata mines, the former of which is an incline fol- 
lowing down the contact to the east, and the two latter tunnels running in 
at or near the contact, in a southerly direction. On the steep hillside, at the 
mouth of the Rock tunnel, stood once a huge outcrop of hard carbonate, 
from which was obtained the first ore of this character found in the region. 
A short distance above the contact, on Dome Hill, is an intrusive sheet 
of Gray Porphyry, which, on the western point of the outcrop, cuts up into 
the White Porphyry, but in California gulch comes actually in contact with 
the limestone, arid at the La Plata mine cuts into it so that a small de- 
tached portion of the limestone is left above this intrusive sheet. It also 
extends up the south slope of Iron Hill, parallel to the contact, and only 
separated from it in places by a thin sheet of green Lingula shales, which 
belong to the Weber Shale formation. At the foot of the steep slope of 
Iron Hill, opposite the Rock mine, the Blue Limestone is laid bare in the 
quarry of the Montgomery claim. 

South slope of iron Hill. The steep north slope of California gulch, from 
here down to the Iron fault, which crosses the gulch at the Garden City 
mine, presents actual outcrops of the lower Paleozoic formations, the Blue 
Limestone, Parting Quartzite, White Limestone, and Lower Quartzite, 
together with an intrusive sheet of Gray or Mottled Porphyry near the 
bottom of the Blue Limestone. In point of fact, these outcrops are covered 
by from six to ten feet of slide material, but are readily seen in the numer- 
ous prospect holes which dot the side of the hill. The dip of the limestone, 
as shown by the various inclines on Iron Hill, varies from 12 to 25, while 
its strike is more nearly north and south than the average strike of the sedi- 
mentary beds throughout the region. In the Iron mine itself the dip shal- 
lows as it is followed into the hill, and becomes, beyond the Tucson shaft, 
nearly horizontal ; while in the Horseshoe shaft, at the head of Nugget 
gulch, which has reached the contact at a depth of 482 feet, the limestone 
is said to have dipped 8 to 10 to the southwest, showing a tendency to a 
synclinal structure in this block of ground, which is still more marked in 
the block next west. The Colonel Sellers shaft and drill-hole, south of this, 
near the mouth of Nugget gulch, had not yet reached the contact. 


North iron Hill. From the Codfish Balls shaft northward to Stray Horse 
gulch the line of the Iron fault is somewhat indefinite, the miners who 
sunk the few shafts not having found any valuable ore bodies at the con- 
tact and having confounded the limestone, which is here bleached quite 
white, with the overlying porphyry. In the angle between the Iron fault 
and the Adelaide cross-fault, as shown by the workings of the Argentine 
and Adelaide mines, the formation dips to the southeast, so that successive 
outcrops of White Limestone and Lower Quartzite are brought to the surface. 
The structure at this point, which will be explained in detail in Part II, 
Chapter II, is still further complicated by the intrusion of minor sheets of 
Gray and White Porphyry, which have split up the Silurian formation, and 
by the crossing of the main sheet of White Porphyry down to the horizon 
of the Parting Quartzite across the basset edges of the Blue Limestone. 
The principal mineralization has here taken place at the contact of this 
White Porphyry with the Parting Quartzite, instead of, as in other cases, 
on the surface of or in the Blue Limestone. 


Carbonate fault. Carbonate fault has a general direction a little more (o 
the east of north than Iron fault. Its upthrow is likewise to the eastward, 
and the displacement has a probable maximum in the bed of California 
gulch, where Silurian beds are proved by shaft developments to come 
in contact with White Porphyry. On the southern slope of Carbonate 
Hill its plane is actually proved in the shafts of the JEtna and Yankee 
Doodle mines. Here its movement is only about two hundred feet; but a 
second fault is found crossing the Glass-Pendery claim, the amount of 
whose movement, which is also an upthrow to the east, is not known, since 
the contact has not been reached on its west side. This fault apparently 
joins the Carbonate fault before reaching California gulch. Northward the 
movement of the Carbonate fault gradually decreases and is partially dis- 
tributed among some smaller faults and folds. In this portion its actual 
plane has not been proved; and it is very possible that it does not extend 
as a continuous fault as far as indicated on the map. Indeed, in the 


Waterloo claim its continuation shows a slight upthrow to the west, so that 
at some point south of that its movement must be null. 

South of California guich. Of the actual rock surface of the southern portion 
of this area, which is deeply buried beneath thick deposits of Lake beds and 
the superincumbent moraines of the Iowa glacier, nothing is as yet definitely 
known. The outlines as given on the map must therefore be regarded as 
theoretical deductions from the structure of the adjoining regions developed 
by actual explorations. That a synclinal fold exists here is well proved, 
and the probable slope of the rock surface beneath the Lake beds would cut 
off the successive sedimentary formations approximately along the lines 
represented on the map. 

In Iowa gulch the few prospect shafts were still in surface material. 
The Black Cat shaft, on the ridge north of the gulch, had been sunk 530 
feet through moraine material and underlying Lake beds. 

In Georgia gulch the developments of the Coon Valley shaft, where a 
drill was supposed to have reached contact at 575 feet, show a thickness of 
200 feet of Wash, 375 feet of Lake beds, and 75 feet of White Porphyry, 
with the contact not yet reached at 650 feet. The Resumption shaft, near 
this, found the same thicknesses of Wash and Lake beds, but had not reached 
the porphyry. In the Zulu King (N-24) and Commercial Drummer 
(U-l), northwest of this, near the top of the ridge overlooking California 
gulch, White Porphyry was found at comparatively shallow depth im- 
mediately under the Wash, showing that beneath Georgia gulch a bay once 
existed in the original Arkansas lake. 

Proof of synclinal fold. The intrusive body of Gray Porphyry between 
White Porphyry and Blue Limestone comes to the surface on the banks of 
Iowa and California gulches, adjoining Dome fault on the west, thus proving 
a westward dip in the underlying formations ; in other words, that they 
basin up to the eastward and that the Dome fault runs along or near the 
axis of a shallow anticlinal fold. It has been reached after passing through 
White Porphyry on the California gulch side by the Bank of France shaft, 
in the angle of the Dome and California faults ; by the City Bank and Oro 
City shafts, higher up the slope ; and by the Vining (N-19), near the fault 


on the crest of the ridge, which reached it after passing through the over- 
lying White Porphyry. 1 

Emmet fault. The Robert Emmet tunnel (O-45) starts in near the 
contact of -Gray Porphyry and overlying White Porphyry. A winze was 
sunk in the tunnel, from which a drift to the west has cut the Emmet fault, 
a short cross-fault, by whose movement a little triangular block of ground 
is lifted up on the westward. Parallel with this fault is a slight anticlinal 
fold, along the axis of which the Columbia tunnel runs in on the contact 
and finds the formation dipping away to the right and left, but more steeply 
to the westward. The Blue Limestone is found in actual rock outcrop on 
the bank of the gulch below this. The Crescentia shaft, a little west of the 
Columbia, had reached the Gray Porphyry under the White Porphyry at a 
depth of 335 feet. It is probable that this body of Gray Porphyry thins 
out to the west of this. 

As to the-exact line of the continuation of the Iron fault on the south 
side of the gulch, if it extends so far, no data have yet been obtained, nor 
can it be definitely stated whether Crescentia shaft is to the east or to the 
west of this line. The dip of the formation west of the Columbia tunnel 
is steep enough to account for the contact not yet having been reached in 
this shaft at a depth of 335 feet. 2 

Graham Park. On the steep west slope of Iron Hill toward Graham gulch 
the White Porphyry is probably at its thickest in this area. The Blind 
Tom shaft has been sunk in it to a very considerable depth, though the 
exact depth could not be ascertained. The City of Paris shaft and bore- 
hole are said to have passed through 200 feet of Lake beds and GOO feet of 
White Porphyry below them. Other shafts on the Carbonate Hill side have 
reached depths of 500 feet and are still in the porphyry. The Devlin shaft, 
however, on the northwest slope of Iron Hill, reached the contact at 200 feet 
and the Highland Mary (P-52) found it at 175 feet. These facts furnish 
a direct evidence of what might have been assumed by analogy, that the 

1 Since the close of field-work contact has been reached in the Viuing at 317 feet and in the Little 
Rosie at 375 feet, in the latter of which the formation is said to dip 30 to the southwest, thus con- 
firming the deductions made from the relations of the two porphyry bodies. 

* Since the close of field-work a westerly-dipping contact is said to have been reached by a drift 
east from the bottom of the Crescentia shaft, at a distance ofSOO feet. 


synclinal structure of Little Stray Horse Park, which is on the direct north- 
ern continuation of this block, continues in modified form to the southward. 
It is very probable, therefore, that the contact rises to the eastward before 
reaching the Iron fault along its entire extent, though it is impossible to say 
at what angle. In the Agassiz, Greenback (053), and adjoining shafts a 
sheet of vein material of relatively small thickness is found dipping to the 
northeast, with White Porphyry on either side. This represents a portion 
of the Blue Limestone which has been split off from the main body by the 
cutting down of the White Porphyry ; that is, the lower sheet of White Por- 
phyry here crosses the Blue Limestone formation at a low angle, leaving 
wedge-shaped portions of the latter above and below it overlapping each 
other. The folding of the Little Stray Horse syncline and subsequent 
erosion have produced a curved line of outcrop, approximately as given 
on the map. The thin streak of blue on the south side of Stray Horse 
gulch represents a thin sheet split off from the main body of Blue Lime- 
stone, which to the northward thickens so as to include the whole of this 
body on Fryer and Yankee Hills ; while here the bulk of the Blue Lime- 
stone is separated from this thin sheet by a great thickness of White Por- 
phyry, probably not less than 600 to 800 feet. 

The Greenback shaft, after passing through Wash and Lake beds and 
10 feet of White Porphyry, found vein material and limestone in a thick- 
ness of . r >5 feet. The Mahala (T-2) passed through 145 feet of overly- 
ing White Porphyry, 10 feet of vein material, and 105 feet of underlying 
White Porphyry. The Agassiz passed through 40 feet of overlying White 
Porphyry, 5 feet of shales, and 30 feet of vein material. The Gone- 
Abroad (T-4) also found vein material, after passing through White Por- 
phyry, at a depth of about seventy-five feet. The Robert Emmet shaft 
(S-3), after passing through 210 feet of Wash and White Porphyry, cut 
50 feet of vein material and passed again into White Porphyry, showing a 
considerable thickening in the body of vein ^material to the northward. An 
actual outcrop of this body of iron is found on the south side of Stray 
Horse gulch, near the Robert Emmet tunnel (S-l 3). The Wolfe Tone shaft 
(T-5), which is about five hundred feet west of the Agassiz, has beeu 


sunk to a depth of over five hundred feet in the White Porphyry, which is 
here underlying the Agassiz deposit, but without reaching the lower Blue 
Limestone. 1 

California gulch. On the west side of the area under consideration rock in 
place has not been fovind south of California gulch, except in the Swamp 
Angel and Jordan (T-14) tunnels on its south bank, which have been run 
for some 400 feet southward on the contact. The Deadbroke (T-16) and 
Rosebud (T-18) have also developed the contact on the north side of the 
gulch, and the J. Harlan shaft has been sunk through Blue Limestone into 
an underlying sheet of Gray or Mottled Porphyry. Higher up the gulch 
the Last Rose of Summer and some adjoining shafts struck slates and sand- 
stones belonging to the Weber Shale formation, which belong to a portion 
of the formation included in the White Porphyry. The Prospect incline, 
starting in at an angle of 23 in the White Porphyry, reached the contact, 
whose angle is somewhat shallower (averaging from 12 to 20), and followed 
it in for a distance of over five hundred feet. At 375 feet from the mouth 
was a sharp fold, possibly accompanied by some displacement, in which the 
contact went down almost perpendicularl} r for about one hundred and 
twenty-five feet, and was found again in its normal position at a distance of 
14 feet beyond in the regular course of the incline. 

The White Limestone is opened in a quarry adjoining the road on the 
north side of California gulch, directly below the Prospect incline. This is 
the only point where the White Limestone is found actually visible on the 
surface in the immediate vicinity of Leadville. The O'Donovan Rossa shaft 
is also in White Limestone, while the Irish Giant, above it, is sunk through 
the same sheet of Mottled Porphyry shown in the J. Harlan, into the under- 
lying half of the Blue Limestone. The shaft (T-46) is also in White 
Limestone, while the adjoining Blind Tom shaft is in White Porphyry on 
the west side of the fault. A second intrusive body of Gray or Mottled 
Porphyry in the White Limestone itself is proved by some small shafts in 
California gulch not indicated on the map, which also show the cropping of 

1 Since the close of field-work the Wolfe Tone shaft has reached vein material and limestone at a 
depth of 625 feet, and after passing through- it struck another body of porphyry, whether belonging to 
the underlying intrusive sheet of Gray Porphyry or White Porphyry is not known. It is probably the 


the upper portion of the Lower Quartzite adjoining the fault. A shaft still 
lower down, opposite the sampling works on the edge of the creek bed, is 
sunk several hundred feet in White Porphyry. 

carbonate Hiii. The area east of Carbonate fault, included in the Car- 
bonate Hill map, will be treated in detail in Part II, Chapter III, and only the 
general features need here be mentioned. The strike of the Blue Lime- 
stone is nearly north and south, bending somewhat to the eastward toward 
the northern end of the hill. Its dip may be taken at an average of 21, 
but is found locally to vary very considerably on account of a series of 
longitudinal waves or folds in the formation. The sheet of Gray or Mottled 
Porphyry within the Blue Limestone is very persistent, and is evidently a 
later intrusion. From data obtained from the few points at which it has 
been proved by underground workings, it is evident that it is not confined 
to any particular horizon, but locally cuts across the beds, sometimes at a 
considerable angle. It is best shown in the Evening Star mine, where it 
seems to be at the base of the Blue Limestone. What is apparently an 
offshoot from it is found at the contact in the Morning Star mine and extend- 
ing up into the overlying White Porphyry, while west of the line of the 
fault in the Forsaken and Henriett mines the main sheet is found cutting 
across the Blue Limestone, and the principal mineralization has taken place 
between it and the portion of the Blue Limestone which underlies it. 

Of the country underlying Stray Horse gulch, Stray Horse Ridge, and 
Little Stray Horse gulch the structural data obtained from explorations are 
somewhat unsatisfactory; but on Fryer Hill the continuation of Carbonate 
fault is found to be a gentle anticlinal fold whose axis runs in a northeast- 
erly direction through the Dunkin ground. 

Little stray Horse synciine. Between Yankee Hill and the crest of Fryer 
Hill, through which also runs a general anticlinal fold, is included a basin 
or synclinal fold in the formation, whose deepest portion underlies Little 
Stray Horse Park. The surface rock in the center of this basin is the main 
sheet of Gray Porphyry, which is separated from the underlying Blue Lime- 
stone by a comparatively thin sheet of White Porphyry. The angle of dip 
of the beds follows the general rule which prevails in the folds in this region 
and is steeper on the east side of this synciine than on the west. 


Eastern rim. The Blue Limestone, which is largely replaced by vein 
material, comes to the surface on the eastern rim of the basin along the 
foot of the steeper slope of Yankee Hill. It is found directly under the 
Wash in the Cordelia Edmonston and adjoining shafts. The Birdie Tribble 
(P-42), at the very edge of the basin, found five feet of porphyry above 
the vein material and limestone. In the shafts of the Kennebec (P 55) 
both Gray and White Porphyry are passed through before reaching the 
limestone, and a sheet of porphyry six feet thick was also cut in the body 
of the limestone. The Chieftain tunnel and incline run in a southeasterly 
direction 360 feet through vein material and limestone, finding the Iron 
fault with granite on its farther side at the end. The limestone here shows 
the effects of a movement against the fault plane, being compressed into short 
sharp folds and much metamorphosed. There is a general tendency, how- 
ever, to dip to the northwest; and it is probable that the extremity of the 
incline is in the White Limestone, although lithological indications are here 
extremely deceptive, owing to the alteration to which the rocks have been 
subjected. The Scooper shaft (P-44), a little to the south of the Chieftain, 
passed through 20 feet of Gray Porphyry and 5 feet of White Porphyry 
before reaching the Blue Limestone. The contact here stands so nearly 
vertical that it was supposed by the superintendent to be a fault. This 
supposition was rendered more probable by the fact that the line of this 
contact runs in a southeasterly direction. It is probably, however, only an 
exceptionally steep dip on this side of the basin. South of this the Del 
Monte (P-45) shaft is in Gray Porphyry. The Hard Cash (P-46) shaft 
is in vein material. The I^airplay (P 34) is still in White Porphyry, below 
the Blue Limestone. The upper White Porphyry, so thin in the Scooper, 
disappears entirely a little farther south, being altogether wanting in the 
Rarus shaft (P-61); or, as it might be considered, it is found entirely 
below the upper sheet of Blue Limestone. 

The fact that the Blue Limestone is split into two sheets by the White 
Porphyry is shown in the shafts east of the Rarus in Stray Horse gulch. 
The Indiana shaft (P-53) finds the limestone directly under the Wash. 
East of this the Young Caribou (P-59) finds White Porphyry under the 


Wash; and the Highland Mary (P-52) and Snowstorm (P-50), after pass- 
ing through White Porphyry, reach the lower sheet of Blue Limestone be- 
neath it. 

Center of basin. Towards the center of the basin a number of shafts have 
been sunk to a considerable depth in the overlying Gray Porphyry, and 
generally find sandstones or black carbonaceous shales at its contact with 
the overlying White Porphyry, but none have as yet reached the Blue 
Limestone. The greatest depths obtained have been in the Little Miami 
(P-58), which went through 269 feet of Gray Porphyry and 30 feet of 
White Porphyry, having a total depth of 396 feet; the Indiana (P-64) 
shaft, 230 feet of Gray Porphyry in a total depth of 330 feet; the El Paso r 
325 feet of Gray Porphyry, having a total depth of 470 feet, and the 
Lickscumdidrix bore-hole (P-68), which went through 400 feet of Gray 
Porphyry without reaching the White Porphyry. The deepest portion of 
the basin is probably somewhere near the latter. 

Western rim. On the western rim of the basin contact has been reached 
in the shafts of the Denver City, Tip -top, and Little Sliver mines, in which 
a varying thickness of black shale and sandstone, belonging to the Weber 
Shale group, has been found at the contact of Gray and White Porphyry. 
The Bangkok (P-77) has penetrated the Gray Porphyry to the underlying- 
White Porphyry, while the Forepaugh (P-76), Cora Bell (P-78), and 
Union Emma (P-79) are still in Gray Porphyry. The Hunkidori shaft, 
in Little Stray Horse gulch, at the southern end of the basin, has already 
reached White Porphyry under the Gray. The Denver City (P-82), 
Wright (P-74), and Shamus O'Brien (P-73) shafts found Gray Porphyry 
under 180, 157, and 165 feet of Wash, and reached the Blue Limestone 
horizon at 234, 320, and 362 feet, respectively, each disclosing about ten 
feet of sandstone and shale, which carried as high as 22 ounces of silver, 
between Gray and White Porphyries. 


As the structure of Fryer Hill will be given in detail in a later chapter, 
it is only necessary here to give a brief outline of its structure as bearing 
on that of the surrounding regions. 


In this area the formations have a general dip to the northeast, while 
along an east and west line they partake of the anticlinal and synclinal 
structure, which is already under discussion. On such a line, as shown 
in sections C and D, it is seen that the formations developed on Fryer Hill 
constitute the western rim of the Little Stray Horse basin, being at the 
same time compressed into a shallow anticlinal and synclinal fold. The axis 
of the anticline runs through the crest of the hill in the ground of the 
Dunkin mine, on a line with the continuation of the Carbonate fault. West 
of this is a broad, shallow synclinal fold, which takes in the ground of the 
Little Chief, Little Pittsburgh, and Chrysolite mines, giving to the outcrop 
of the Blue Limestone, as shown on the map, the form of an S. In the 
western portion of the Chrysolite mine ground, successively lower sheets 
of the lower White Porphyry, White Limestone, and Lower Quartzite 
come to the surface along the crest of an anticlinal fold, on whose western 
side, so far as the meager data obtained show, these beds dip steeply under 
the Wash and Lake beds which form the rnesa-like surface of North 
Leadville. The difficulty of reading the geological structure of this area, 
which in the above brief statement seems simple enough, is enhanced by 
a variety of causes. In the first place, here, as in Little Stray Horse 
Park, there are no outcrops of rock in place, the rock surface being buried 
beneath about 50 to 100 feet of Wash. The data have therefore to be 
entirely obtained from shafts, and cannot be intelligently considered until 
they have been thoroughly mapped. Secondly, the replacement action has 
proceeded so far that practically no limestone is left, its whole mass having 
been replaced by vein material. Thirdly, this mass has been split up 
locally into two or more distinct sheets by the intrusion of White Porphyry. 
Fourthly, the lower sheet of White Porphyry is cutting across the forma- 
tion; and, southwest of a line drawn diagonally through the corners of the 
Fryer Hill map, a wedge-shaped portion of the Blue Limestone is left below 
this sheet. Fifthly, there are later intrusions of Gray Porphyry extremely 
difficult to trace, as in their decomposed state they are scarcely distinguish- 
able from the White Porphyry. An interrupted dike of this rock runs 
through the middle of the area in an east and west direction; and an intru- 
sive sheet cuts diagonally across the White Limestone up into the lower 


sheet of White Porphyry, and on the north slope of Carbonate Hill into the 
Blue Limestone. This Gray Porphyry is exposed in the Vulture No. 2 
workings of the Chrysolite mine, in the No. 5 shaft and drifts connecting it 
with No. 1 of the New Discovery mine, and on Carbonate Hill in the lower 
workings of the Waterloo and Henriett claims. The porphyry dike is seen 
in the workings of the Chrysolite, Little Chief, Little Pittsburgh, Amie, Big 
Pittsburgh, Hibernia, and Lee mines. The White Limestone has been 
reached in the Amie No. 2 shaft, New Discovery No. 6, and found at the 
surface under the Wash in the shafts of the Fairview, All Right, and Kit 
Carson, and in the Chrysolite No. 6 (S-51), while the Lida shaft (S-52), 
near Gumming & Finn's smelter, and the Little Eva (S 53) reach the 
Lower Quartzite below the Wash, the former finding a small body of White 
Porphyry included in it. 


North of Evans gulch the geological structure, although probably more 
simple, is more difficult to read, owing to the thickness of loose detrital mate- 
rial above the rock surface and the relatively small amount of underground 
exploration. The North Evans moraine covers an area, widening towards 
its lower end, which but few miners have been enterprising enough to pen- 
etrate to the rock surface beneath, while on Prospect Mountain itself the 
Weber formations and the various porphyry bodies, of which it is mainly com- 
posed, present but few definitely distinctive characters by which to guide 
the geologist. In this region faulting action has apparently entirely ceased, 
and the structure is that of a somewhat irregular system of anticlinal and 
synclinal folds, whose axes run in such varying directions that it is difficult 
to deduce from them a satisfactory system. Sections A and B, Atlas Sheets 
XV and XVI, which run east and west, and Sections M and N, Atlas Sheet 
XXII, which run north and south, give a graphic delineation of the system 
of folds at right angles to either. 

crest of the ridge. To the north and east the White Porphyry gradually 
thins out and the Gray Porphyry comes in contact with the Blue Lime- 
stone, while above this a sheet of Mount Zion Porphyry rapidly thickens 
and reaches its maximum on the north side of the Prospect Mountain, facing 



the East Arkansas Valley. West of the summit of Prospect Mountain the 
structure is that of a broad anticlinal and synclinal fold. On this line, by 
a deeper erosion at the head of the north fork of Little Evans, the body of 
Mount Zion Porphyry has been exposed, to be covered again farther west 
by portions of the Weber Shales arid Weber Grits which have escaped 
erosion on the top of Canterbury Hill, while at the foot of the steep slope* 
in the valley of the east fork of the Arkansas the Blue Limestone comes to- 
the surface beneath the overlying Gray and White Porphyries. The Weber 
Shales, which are brought to the surface by erosion, on the east side of 
the Mount Zion Porphyry, are shown in the Esmeralda, Spotted Tail 
(1-2), Little Maud (1-3), and Peru (1-5). The Thin Space (1-6) shaft 
penetrated them to the underlying Mount Zion Porphyry, and the Texas 
Ranger and Texas Boy's Chance, together with the intervening shafts, are 
in the outcrop of Mount Zion Porphyry, which is traced as far west as- 
the Liberator. 

Southern slope. Along the foot of the steep southern slope of the mountain 
runs an anticlinal fold with an east-and-west axis, whose culminating point, 
as shown in Section N, is at the forks of Little Evans gulch. Between this 
and the top of the ridge is a shallow syncline, along whose axis a portion 
of the Weber Grits is left above the Weber Shales. The Gray Porphyry 
underlying the Weber Shales on the west side of this syncline is developed 
by the Brick Top, Bosco, Moose, and neighboring shafts. Towards the 
north fork of Little Evans the Hecla and Mountain Lion shafts and the 
Boettcher (Q-20) and adjoining (Q-19) tunnels are in the Weber Shales; 
the Geneva Lake (Q-3), Mary Ella (Q-4), Katie Sullivan (Q-ll), and 
Buncombe (Q-13) in the underlying Gray Porphyry. 

On Canterbury Hill the Garland (Q-33), Little Willie (Q-49), and 
adjoining shafts are also in the Weber Shales, on the south side of the syn- 
cline ; likewise the Maryland, which develops the commencement of the 
body of Mount Zion Porphyry, here only five feet in thickness The 
Resumption (Q-60) shaft is in the Weber Grits, in the middle cf the syn- 
cline. The Cardinal (Q-39) shaft finds a thin detached body of Weber 
Shales between Gray and White Porphyries 


The Great Prince and Minneapolis, on the north side of the syncline, 
develop the Mount Zion Porphyry under the Weber Shales, of which in the 
latter shaft a bed 30 feet thick seems to be included within the body of 
Mount Zion Porphyry Between the Princeton (Q-52) and Little Blonde 
tunnels and the St. Louis shaft the data furnished by intervening shafts show 
the. existence of a second minor syncline. The St. Louis reaches the lime- 
stone after passing through 45 feet of Gray Porphyry and 30 feet of White 
Porphyry. The Mary Ann shafts (Q-51 and Q-56) find White Porphyry 
at the surface on the crest of a minor anticline. The shafts Q-45 and 
Q-46 are in Gray Porphyry at the surface, while the Little Blonde arid 
Princeton tunnels develop a considerable body of iron-stained chert, re- 
placing the Blue Limestone and dipping to the north under the White 

Little Evans anticline. Immediately below these two tunnels is the apex 
of the Little Evans anticline, whose main axis runs east and west. It is 
also connected with the Yankee Hill anticline by a fold running south- 
easterly and with the Big Evans anticline by one running southwesterly, 
between which is included the northern extension of the Little Stray Horse 
syncline. The lowest formation exposed on the crest of the Little Evans anti- 
cline is the Lower Quartzite, which is found below the Wash in the Luck- 
now shaft (Q-54). The Norcom (Q-55) shaft, a little north, finds the 
White Limestone dipping northward, and the Little Clara (Q-63), south of 
this, penetrates the White Limestone to the underlying quartzite. A little; 
northwest of this the Lac-la-Belle finds Blue Limestone beneath the Wash, 

The axis of the east and west fold, 'which sinks to the eastward, can 
be traced in a line of shafts from the Lucknow to the Uncle Sam. The 
Catawba tunnel (Q-41) runs in on the Blue Limestone just above the Part- 
ing Quartzite. The Carbonate No. 2 (Q-37) shaft is sunk through a body 
of Gray Porphyry, which is included in the Blue Limestone, into the Blue 
Limestone below, at a depth of 140 feet. The Swing tunnel (Q 42) and 
the Copenhagen (Q-43) and Carbonate King (Q-36) shafts are in the Blue 
Limestone'on the south side of the fold. In the Hancock (Q-31) and Prov- 
idence (Q-32) shafts, on the crest of the fold, Blue Limestone dips with it 
eastward. The Pacific shaft (Q-35) shows a southward dip in the Gray 


Porphyry overlying the Blue Limestone. The Columbia shaft, between 
the forks of Little Evans, penetrates 30 feet of Gray Porphyry and 100 feet 
of Blue Limestone to the Parting Quartzite beneath. The Humboldt and 
other shafts between the last mentioned and the Uncle Sam are all in Gray 
Porphyry. At the Uncle Sam shaft the White Porphyry comes to the 
surface in the crest of the anticlinal fold, whose axis here rises so that for a 
short distance the porphyry has been eroded off it. The Uncle Sam shaft 
has been sunk for a depth of 420 feet, passing through 100 feet of White 
Porphyry, the underlying Blue Limestone, Parting Quartzite, and White 
Limestone, and extends 40 feet into the Lower Quartzite, while the Uncle 
Sam tunnel has been run 250 feet into the overlying Gray Porphyry, and 
the Powhattan (Q-7) shaft adjoining was sunk through White Porphyry 
into the Blue Limestone. The Powhattan (Q-9), Home (Q-6), Eaton 
(Q 10), and others on the hill above are in Gray Porphyry. 

Yankee Hiii anticline. Of the anticlinal ridge connecting the Little Evans 
with the Yankee Hill anticline, few data have been obtained. The Little 
Hoosier, Abe Lincoln, and shafts P-29 and P-38 have penetrated the 
Wash to the underlying Gray Porphyry, in which the first named has been 
sunk 170 feet, the moraine material at this point being 120 feet deep. The 
shaft P-39 has reached the Blue Limestone beneath the Wash, and the 
Chicago Boy (P-67) passes through the Parting Quartzite into the lower 
sheet of White Porphyry. This lower sheet of White Porphyry has not 
been found north of this point, and is supposed to wedge out. 

Little stray Horse synciine. In the northern continuation of the Little Stray 
Horse synciine the Buffalo shaft and drill-hole, on the Evans moraine ridge, 
is said to have reached a depth of 450 feet and is still in Gray Porphyry; 
and the shaft S-10 is also in Gray Porphyry. No other data could be 
obtained as to the depth of this basin, so that it can only be said that in its 
center the contact is probably 500 feet deep at least. 

Big Evans anticline. Of the Big Evans anticline, which is a continuation 
to the northward of that shown on the west edge of Fryer Hill, data are 
still more meager. The Argo (R-5) shaft finds White Porphyry beneath 
the Wash and is sunk into the underlying Blue Limestone. Adjoining this 
on the east is the Douglas (R-4a) shaft, and on the north the R-4 shaft, 


each in White Porphyry on either side of what is supposed to be the ridge 
of Blue Limestone connecting this anticline with South Evans anticline. 
The Third Term (S 44) bore-hole, just across Evans gulch from the Gum- 
ming & Finn smelter, passed through 170 feet of Wash -into the Lower 
Quartzite, in which it found a small body of White Porphyry, supposed to 
be the same as that already mentioned as found in the Lida (S-52) shaft, 
on the other side of the anticline. The outcrop of Archean indicated on 
the map has not been proved by any shaft, but is simply a theoretical de- 
duction from the dip of the beds, the rock surface being buried beneath one 
to two hundred feet of Wash. On the southwest slope of this anticline the 
Mystic and Silver Pilot (R-8) have been sunk a short distance in the over- 
lying Gray Porphyry. The Oolite (S-57) shaft passed through 100 feet 
of Gray Porphyry and 15 feet of White Porphyry, reaching a considerable 
body of vein material and chert, in which were found fossils characteristic 
of the Blue Limestone horizon. The Sequa shaft (S-58), about eleven 
hundred feet west of this, reached a depth of 280 feet, still in Gray Porphyry, 
showing that the actual contact must be at a still greater depth, and thus 
proving the southwestern dip on this side of the anticline and a synclinal 
fold to the west. 


General structure. From the foot of Carbonate and Fryer Hills extends a 
broad, flat, mesa-like ridge, sloping at a regular angle of about two and a 
half degrees to the Arkansas Valley. This even surface is doubtless con- 
formable with the surface of the stratified Lake beds which underlie it, over 
which rearranged moraine material or Wash has been spread out with com- 
parative uniformity by the action of water. The relics of the moraines 
which were left by the Big Evans glacier are found in the ridge which ex- 
tends from the west end of Fryer Hill to Capitol Hill ; also, in James Ridge, 
adjoining the mouth of Big Evans gulch, and in a smaller ridge between 
the two, below North Leadville. No shaft has yet reached the rock sur- 
face beneath these recent accumulations of detrital material. The outcrops 
indicated on the map, and the basin character of the area, as shown in cross- 
sections, are therefore, in one sense, purely theoretical. As they have been 
determined, however, after a careful consideration of all the known facts 


and probabilities, it is well to state somewhat in detail the grounds on which 
the existence of a synclinal basin is rendered probable. The first argument 
in its favor is that of analogy, drawn from the existence of a synclinal basin 
adjoining it on the east, in Little Stray Horse Park, which evidently con- 
tinues southward through the block of ground between the Carbonate and 
Iron-Dome faults. The facts to support this argument, viz, the proof of an 
actual dip towards the center of the basin from either side, are as follows : 
Eastern rim of basin. Fii'st, a western dip in the overlying beds on the 
west side of the Big Evans anticline is shown by the developments of the 
Oolite and Sequa shafts. Secondly, the Bob Ingersoll shaft and drill-hole, 
on the moraine ridge west of Fryer Hill, in East Ninth street, after passing 
through moraine material and Lake beds, are said to have penetrated nearly 
three hundred feet of White Porphyry. This shaft is southwest of the line 
where the White Porphyry cuts down below the horizon of the Blue Lime- 
stone. It is probable, therefore, that the porphyry cut in this shaft belongs 
to the upper sheet above the Blue Limestone, and the fact that so great a 
depth as 300 feet has been reached without finding contact indicates a very 
steep dip to the westward. The owners of the American Eagle shaft, at 
the west base of Fairview Hill, state that the limestone -in their workings, 
which the dump shows to be White Limestone, dips both eastward and 
westward, which would show that there is an anticlinal fold here. 1 Third, 
along the west base of Carbonate Hill, the Pocahontas (T-40), Weldon 
(T-41), Rough and Ready, and other shafts have been sunk to a consider- 
able depth in the White Porphyry beneath the Wash, and the California 
tunnel is also in White Porphyry until it reaches the Blue Limestone 
beyond the fault. This White Porphyry can be no other than that which 
overlies the Blue Limestone, since in this region no considerable body of 
White Porphyry is known to exist below this horizon; moreover, in the 
Niles- Augusta, Wild Cat, Washbume, and other mines, as will be explained 
in the detailed chapter on Carbonate Hill, there are indications of a prevailing 
western dip to the formations west of Carbonate fault This summarizes 
the evidence of westerly dipping beds on the east side of the synclinal basin. 

1 Since the close of field-work, Mr. E. N. Clark, superintendent of the Chrysolite mine, states 
that the extreme west workings of that mine show the Lower Quartzitc and White Limestone to be 
dipping to the westward. 


western dm. On the western side, in the little canon at the mouth of the 
east fork of the Arkansas, adjoining the west end of James Ridge, the Lower 
Quartzite is exposed in considerable thickness at the surface, dipping at an 
angle of 8 to 10 to the southeast, and some workings to the east of this, 
along the northern edge of James Ridge, are said to have disclosed the over- 
lying White Limestone. The Peoria shaft, on James Ridge (not indicated on 
the map), may be expected to afford further data as to the actual line of out- 
crops of the formations and what portion of them have escaped erosion, 
when it reaches the rock surface. At the time of writing this shaft had a 
depth of 375 feet and was still in the marl of the Lake beds. 

From these meager data and from the probable thickness of Lake beds 
and the angle cf dip of the underlying formations the line of outcrops of 
the western rim of this basin have been constructed. While, therefore, the 
fact that a synclinal basin exists beneath this area seems fairly well estab- 
lished by the evidence just given, there is only a possibility that the line of 
outcrops given on the map will be found by future exploration to be 
strictly correct. They are dependent on two as yet unknown quantities : 
first, the angle of dip of the formations on either side of the basin, and, 
secondly, the amount of erosion which had taken place before the Lake beds 
had been deposited, or, what amounts practically to the same thing, the 
thickness of the Lake bed deposits which now underlie Leadville. 


The detailed description given above of the geology of the Leadville 
area can perhaps best be summarized in a consideration of the various sec- 
tions which accompany the map, and in which this structure is graphically 
delineated. For its better comprehension the reader is requested to place 
these sections one above the other in the order indicated by their letters, 
commencing at the top. The first nine sections (A to I) are on east and 
west lines, approximately parallel with each other. These sections, being 
in general across the strike and more or less at right angles to the fault 
planes, show not only the amount of displacement occasioned by these 
faults, but the longitudinal folds into which strata have been compressed, 


and which are more or less intimately connected with the faults. The 
other seven sections (J to P) run north and south and give the effects of 
lateral -pressure. As these are more or less parallel to the fault planes, 
they intersect the latter generally at an acute angle, and the angle of inter- 
section is often much lower than the actual slope of the fault plane. 

In representing the slope of the fault planes, in all cases where there 
were no data from actual developments it has been given as inclining toward 
the hanging-wall side at an average angle of 75, and when cut diagonally 
by the plane of the section the angle of intersection was calculated from 
these premises. As all these sections are carefully constructed to scale and 
have a common base line, which is taken at 9,000 feet above sea-level, they 
represent with a high degree of accuracy the surface of the country and the 
relative thickness of the different sedimentary bodies, and in less degree that 
of the porphyry bodies, as far as can be deduced from their surface outcrops. 
In order to show as far as possible the data from which these sections 
have been constructed, the various shafts on or in close proximity to the 
plane of each have been indicated on the sections by lines running below 
the surface to show the depth to which their explorations have reached, 
full lines indicating those on the section plane, dotted lines those near it. 
The relative frequency of these shafts is therefore an indication of the com- 
parative accuracy of the different portions of the section. It must, how- 
ever, be borne in mind that the underground structure has been arrived at 
not solely by consideration of the shafts on the actual line of the section, 
but also by the consideration of the data obtained from the exploration of 
shafts over a comparatively large area, which afford grounds from which the 
theoretical structure may be deduced. 

section A. Section A runs along Prospect Mountain ridge a little 
south of its' crest and crosses diagonally the valley of the east fork of the 
Arkansas near its mouth. Its line lies entirely north of the extreme limits 
to which the movements of the faults have been traced. Its structure lines 
contrast strongly with those of the other sections on account of the broad 
and regular curves. This contrast is probably greater than that existing 
in nature from the fact that actual data from beneath the surface along this 
line are almost entirely wanting, and the underground outlines are simply 


theoretical prolongations of observed dips. The depth of the Blue Lime- 
stone horizon at the east end of the section is probably a maximum. 
Analogy renders it probable that the eastward dip shallows, and it is pos- 
sible even that the beds rise somewhat towards the Mosquito fault. This 
remark applies equally to the corresponding points in the next four sections. 
The sheet of Sacramento Porphyry is represented here between the Weber 
Shales and Weber Grits, the horizon at which it occurs on the crest of the 
range east of the Mosquito fault, since it is fair to suppose that the sheet 
extended as far west as indicated on the sections. The thickness of the 
Mount Zion Porphyry on the west slope of Prospect Mountain can only 
be a matter of conjecture ; it is fair to infer that it reaches at least 700 feet 
in its maximum development. The extension of Lake beds as far north as 
the line of this section in the Arkansas Valley is proved by excavations on 
the north bank of the stream. At the western end of the section the shore 
line against the Archean is shown in the abrupt termination of the Lower 
Quartzite. This would have been more striking had the section line been 
placed a little farther north, when the White Limestone Avould have been 
found to come in actual contact with the Archean. 

Section B. Section B follows in its western course the bed of Evans 
gulch ; then, cutting across the southern spur of Prospect Mountain below 
the Prospect amphitheater, follows approximately the line of Little Evans 
gulch, and, crossing the two gulches diagonally just above their junction, 1 
runs out on to the mesa at the intersection of the railroad line with Evans 
gulch. It thus shows portions of the Evans north moraine and, at its 
crossing of Evans gulch, the supposed shore-line of Arkansas lake. The 
eastern half of the section shows practically the same structure as Sec- 
tion A, except that the Weston fault comes in in the axis of the broad 
anticline. In its western half it cuts across the northern extension of the 
Yankee Hill and the Big Evans anticlines, and of the included Little Stray 
Horse syncline; and west of this shows the probable slope of the beds in 
the syncline beneath the mesa, as proved by the explorations of the Oolite 
and Sequa shafts ; also, the White Porphyry, cutting across the Blue Lime- 

1 On the section its intersection with Little Evans gulch is -wrongly marked " ditch." 


stone where the northwest and southeast zone through Fryer Hill would 
intersect the section-plane. 

Section c Section C runs through the crest of Little Ellen Hill in a 
direction a little north of west, crossing the South Evans anticline opposite 
the western point of the hill; thence following the south bank of Big Evans 
gulch across the north slope of Fryer Hill, it passes through the mesa 
just north of the railroad station in North Leadville. It thus shows at its 
east end the movement of the Mosquito fault; and, between this and the 
mouth of South Evans gulch, the same regular easterly dipping beds 
seen on the previous sections, slightly displaced by the movement of Ball 
Mountain fault. On either side of its intersection with South Evans 
gulch the considerable accumulation of recent material (r) represents the 
moraine left by the Evans glacier. The White Porphyry above the Blue 
Limestone, which in the preceding sections had thinned out near the crest 
of the fold, is now supposed to extend back to the Mosquito fault, but in a 
comparatively thin sheet; while in the crest of the South Evans anticline 
the dike cut by the Silver Tooth bore-hole is represented as the source of 
the White Porphyry sheet immediately overlying the granite. The plane 
of the section intersects that of the Weston fault at its junction with the 
Colorado Prince fault, and, on the theory of an inverted dip to the latter 
(assumed from the fact that granite overlies White Porphyry in the Boulder 
incline), would also intersect the plane of the latter at the angle given in 
the section. Beyond Weston fault the upward roll in the beds at the 
Great Hope mine is graphically shown, arid the syncline included between 
this ridge and the crest of Yankee Hill. The section line passes north 
of the summit of this hill, and beyond this point shows the increasing 
thickness of the Wash or moraine material left by the Evans glacier. Its 
intersection with the Iron fault is at the northern extremity of that fault, 
whose movement is deduced, from data furnished by the Little Stella and 
J. B. Grant shafts, on either side. Beyond this it passes through the Little 
Stray Horse syncline, the anticline in the Dunkin ground, and the syn- 
cline on the north side of Fryer Hill. The relative thinning out of the 
Blue Limestone on Fryer Hill, where it is entirely replaced by vein ma- 
terial, is to be remarked. This replacement, which is shown by a cross- 


marking, is not indicated in the Blue Limestone in the basin of Little 
Stray Horse Park, not because there is any reason to suppose that it does 
not exist there, but simply because explorations have not proved its exist- 
ence and in drawing sections the practice has been established of only in- 
dicating replacement where it has been actually proved. The steep slope 
given to the beds west of Fryer Hill, as they pass under the western syn- 
cline, is deduced from data obtained on the line of the next following 
section. It will be observed that the intersection of the line along which 
the White Porphyry cuts across the Blue Limestone is here farther -east 
than in the preceding section, and that the eastern extent of the lower 
White Porphyry body is considerably greater is proved by actual develop- 

Section D. Section D starts from the same point on the eastern edge of 
the map as the preceding, but follows a line slightly divergent from it, run- 
ning due west. The planes of the two sections are so close together that 
it will be only necessary to mention the points in which the structure of 
the latter differs. In the South Evans anticline it shows the irregular 
intrusive sheet of porphyry at the base of the Blue Limestone, developed 
in the Last Chance shaft, and the lower sheet of White Porphyry, cutting 
up into the Lower Quartzite and splitting off a portion _of it, as shown in 
the Hoosier Girl (Gr-44). The intersection with the Colorado Prince fault 
at an acute angle renders the representation of the western slope of the 
South Evans anticline somewhat less simple. Its line passes through the 
crest of Yankee Hill, showing the replacement of the Blue Limestone in the 
Greenwood and Little Champion shafts, and, on the eastern rim of the Little 
Stray Horse syncline, the steep dip of the contact which is developed in 
the Scooper shaft. At Fryer Hill it passes along the bed of Little Stray 
Horse gulch, showing that the Blue Limestone horizon has there been eroded 
off. It likewise passes through the Bob Ingersoll shaft, and shows the steep 
dip theoretically required on the western slope of the anticline by the 
development of this shaft. 

section E. Section E runs due east and west along the parallel of lati- 
tude 39 15', which forms the middle of the map, and is but a compara- 
tively short distance south of the line of the two previous sections. On the 


eastern end it shows the Mosquito fault and a patch of Lower Quartzite 
left to the east of it, on the slope of West Dyer Mountain. In the block 
between Mosquito and Ball Mountain faults the easterly dip prevails; but 
in the neighborhood of the latter fault the influence of the anticline at the 
north foot of Ball Mountain is seen in a slight curvature of the beds. Be- 
tween Ball Mountain and Weston faults the Colorado Prince fault cuts 
through the southern extension of the South Evans anticline ; and the sec- 
tion shows a minor anticlinal and synclinal structure between this and the 
Weston fault, which is shown by .the developments of the Highland Chief 
and Lowland Chief shafts and the Chemung tunnel. Replacement in the 
Highland Chief mine is supposed to have extended through the entire 
thickness of the Blue Limestone horizon and to have been influenced by 
the dike of Gray Porphyry which is shown in that mine. The recent for- 
mation (r) east of the Highland Chief mine is a portion of the moraine 
left by the South Evans glacier on the shoulder now called Idaho Park. 
In the block west of Weston fault the shallow anticlinal and svnclinal 


structure developed in the two previous sections is supposed to extend into 
the plane of this section, the underground data confirming this idea as 
far as they go. The plane of the section is very nearly coincident with 
the line of the Breece cross-fault, which, however, in its curves crosses it 
at an extremely acute angle. The projection of the intersection of these 
two planes, as shown on the section, is a line cutting the surface between 
the Breece Iron and Louisville shafts, which has a certain parallelism with 
the formation lines, with which it might be confounded. The plane of the 
section passes just north of the extremity of the Mike fault, whose move- 
ment is therefore not shown. The body of Adelaide Porphyry is repre- 
sented as coming up across the Lower Quartzite and White Limestone, and 
then spreading out, sending an offshoot between the beds of the latter. At 
this point the plane of the section crosses the moraine ridge, north of 
Adelaide Park, forming a portion of the Evans south moraine. In the 
block between Iron and Carbonate faults the line of section illustrates 
plainly the synclinal structure and the splitting of the Blue Limestone into 
two sheets, as proved in the Cyclops, Gone-Abroad, and adjoining shafts. 
On the west slope of Carbonate Hill the section passes through the 


upper Henriett workings and the lower workings of the Waterloo claim, 
and shows the anticlinal axis, which very nearly corresponds with the Car- 
bonate fault, in the latter. This axis, as already stated, is found to coincide 
with the line of the fault farther north; and it is possible that on the line 
of the section the fault movement may have already died out, since its actual 
plane has not been proved. 1 Of the synclinal basin under Leadvllle in the 
line of this section the depth and angle of the formations on its eastern rim 
are deduced from actual data, which are riot, it is true, as complete as could 
be wished The location of the western rim, however, is more theoretical. 
Section F. Section F, on a slightly broken line, passes through the crest 
of East Ball and Ball Mountains, from the latter across the slope of Breece 
Hill to the head of Nugget gulch, through the middle of Iron Hill, and 
along the bed of California gulch, into the mesa country. East of the Mos- 
quito fault it shows a patch of Lower Quartzite left on the crest of East 
Ball Mountain. Between Mosquito faultand Ball Mountain fault it shows the 
development of White Porphyry in the lower horizons and its comparative 
absence above the Blue Limestone ; between the converging Ball Mount- 
ain and Weston faults, the great development of Pyritiferous Porphyry and 
the probable continuation of the numerous sheets of White Porphyry in 
the lower horizons. The anticlinal structure shown in the eastern portion 
of this block represents the supposed influence of the anticline observed 
at the north base of Ball Mountain, which, owing to the curvature of the 
line of Ball Mountain fault, is the proper continuation of this portion of 
the area. The distribution and thickness of the numerous bodies of por- 
phyry in the latter block are deduced mainly from the data obtained in the 
adjoining regions, since along the actual plane of the section, as will be evi- 
dent from its examination, there are few data obtained from underground 
workings. In the next block, between Weston and Pilot faults, a body of 
Pyritiferous Porphyry is shown in section, whose thickness is largely a mat- 
ter of conjecture, the only direct evidence being that of the Cumberland 
shaft, near its northern edge, where it is 450 feet. As the plane of the sec- 
tion probably cuts through the thickest portion of the body, the thickness of 

1 Later developments render it probable that the sheet of Gray Porphyry is in the Blue Limestone, 
above the line of Carbonate fault, instead of at its base, as shown in the section. * 


600 feet given must be considered a conservative estimate. The underly- 
ing White Porphyry is shown as disappearing in the middle of this body, 
since the latter is supposed to connect with the lower body in California, as 
shown in Section L. A slight anticlinal structure is shown in the sedi- 
mentary beds beneath the porphyry, as a probable connection between the 
Great Hope anticline on the north and that under Printer Boy Hill on the 
south. The block between Pilot and Mike faults, it is seen, is practically a 
wedge-shaped mass which has slipped down between the two faults. In 
this area is the intersection of the line of cross-cutting White Porphyry, 
which is therefore indicated here as spreading out under the Blue Lime- 
stone. On Iron Hill the line of section passes through the workings of 
the Iron mine; and the data down to the horizon of the Blue Limestone are 
derived from actual exploration. The transverse body of Gray Porphyry 
developed in these workings is supposed to be an offshoot from the intrusive 
sheet at the base of the Blue Limestone; it should not have been represented 
as actually projecting into the White Porphyry. 

West of the Iron fault the section crosses what is probably the greatest 
thickness of White Porphyry left above the Blue Limestone, but the depth 
of the latter immediately adjoining the fault is, as already stated, purely 
theoretical and given as a probable maximum. 

section G. Section G follows also a slightly broken line along the south 
slope of Ball Mountain, through Green Mountain and the head of Califor- 
nia gulch, and then along the northern edge of Dome Ridge. At its eastern 
end it crosses diagonally the South Dyer fault. Between Mosquito and 
Ball Mountain faults the development of White Porphyry in the lower 
horizons is even more striking than in the preceding section. Between Ball 
Mountain and Weston fault the distribution of the porphyry bodies is 
similar to that in the preceding section, but the Pyritiferous Porphyry is 
supposed to be thinning out to the southward. 

Between Weston and Pilot faults the section shows a probable vent of 
the lower bod}* of Pyritiferous Porphyry, which is known to cut across 
the strata, and probably comes through the Archean in this vicinity. In 
the wedge-shaped mass between Pilot and Mike faults a sheet of Pyritifer- 
ous Porphyry is supposed to extend between the White and Blue Lime- 


stones, as a continuation of the lower sheet 0*" Pyritiferous Porphyry which 
forms the bed of California gulch above the Pilot fault. West of Mike 
fault the contact has been carried back at the angle shown in (he develop- 
ments of the Oro La Plata mine, and the intrusive body of Gray Porphyry, 
which cuts across it at the mouth of the Oro La Plata tunnel, is repre- 
sented as probably thinning out to the eastward. It is possible, however, 
that the contact basins up toward the Mike fault, as it does on Iron Hill 
and the Gray Porphyry sheet may have an underground connection with 
the Printer Boy Porphyry, which it somewhat resembles, and both come 
up through the same channel or vent, West of the Dome fault the west- 
ward slope of the beds in the lifted-up block of ground between the Robert 
Emmet and . Iron faults is shown. Beyond the Iron fault the only data 
obtained from shafts are the relative positions of the Wash, Lake beds, and 
underlying porphyry. 

Section H. Section II is taken along a straight line running from the 
bed of Iowa gulch, on the eastern border of the map, through Printer Boy 
Hill and down the bed of Georgia gulch. The three eastern blocks do not 
differ sensibly from those of the preceding section, except that, as proved 
by actual developments on Printer Boy Hill, the White Porphyry above 
the Blue Limestone is exceedingly thick, for which reason its thickness in 
the adjoining block to the eastward is proportionally increased over that of 
Section G. The cross-cutting of the White Porphyry comes just west of 
the Weston fault, and, though entirely below the surface, is not wholly theo- 
retical here, but proved by developments of adjoining mines. The anticli- 
nal structure of Printer Boy Hill, the intrusive sheets of porphyry, and the 
.three vertical dikes are also proved by actual observation. The conver- 
gence of the planes of the Mike and Pilot faults is a theoretical deduction 
founded on the theory of fault planes by observers in other parts of the 
world. The existence of Lake beds at this height is proved by the data 
afforded by the explorations of the Printer Boy mine. The depth shown 
for the Blue Limestone here may possibly be too great, since it is obtained 
by carrying back the angle of dip at the surface near the Dome fault, and 
it is very possible that the beds turn upwards towards the Mike and Pilot 
faults, under the influence of the Printer Boy anticline. The thickness of 


Lake beds below Dome fault and the depth of the contact are derived 
from actut.l data as far west as the Coon Valley; beyond that they are the- 
oretical deductions. 

Section i. Section I is a broken line following, as near as may be, the 
crest of Long and Derry Ridge from West Sheridan Mountain westward. 
It shows the beds left on the crest of West Sheridan ; the anticlinal struct- 
ure developed on Long and Derry Hill, between Mosquito and Weston 
faults; the uplifted block of ground between Weston and Union faults; 
the outcrop of Blue Limestone and general character of its replacement in 
the Long and Derry mines and the great thickness of the White Porphyry 
above it (the cross-cutting of the lower sheet of White Porphyry must 
have occurred in the part eroded off) ; the transverse dikes of Gray Por- 
phyry, and the intrusive sheets of Green Porphyry between the White 
Limestone and Lower Quartzite; and, west of Mike fault, the outcrops of the 
Blue Limestone shown in the Hoodoo and Echo shafts, and the supposed 
form of the body of Josephine Porphyry, between it and the overlying 
White Porphyry. The depth assigned the contact adjoining the Mike fault 
may be too great, as in the previous section, since it is possible that the beds 
rise toward an anticlinal fold. In the actual plane of the section the Lake 
beds are not shown to reach as high up as they do on Section H; but their 
extent on a line immediately north and south of this plane would be equal 
to that of the former. West of the Hoodoo outcrop is indicated the anti- 
cline which forms the southern continuation of the Dome fault, beyond 
which a syncline must exist, as an extension of the synclinal basin proved, 
to the north; but what portion of the synclinal beds involved in these folds 
has escaped erosion is a matter of pure speculation. 

Perhaps the most suggestive teaching afforded by the north-and-south 
sections is the graphic representation they give of the relative character and 
amount of Glacial and Post-Glacial erosion. They afford successive cross- 
sections of the various spurs represented on the map from the summit down 
to the mesa below. Where Lake beds still exist, the rock surface below 
them is the result of erosion in the earlier portion of the Glacial period. 
The rock surface beneath the moraine material (r), whether in its original 
ridges or rearranged, is probably practically the same as it was at the close 


of the Glacial epoch, while the sky-line of each section represents the final 
form which water has given to the surface left at the close of the Glacial 
epoch, whether it be rock or detritus. 

section j. In Section J, which crosses Prospect Mountain ridge, the 
lower portion of Little Ellen Hill, Ball Mountain, and upper Long and 
Deny Ridge, are seen the main depressions made by the Evans, South 
Evans, and Iowa glaciers, the outlines of their beds somewhat rounded off 
by Post-Glacial erosion. The formations are seen to have three broad 
undulations rather than folds, the two southern of which are broken by 

Section K. In Section K, which passes through Prospect Mountain, 
Breece Hill, the head of California gulch, and Long and Derry Hill, the 
two Evans glaciers had come together in one broad sheet of ice a mile in 
width and not less than six hundred feet in thickness. Of the moraine 
material still remaining here, a portion evidently belongs to the lateral 
moraines, and in the middle is left a relic of the medial moraine formed by 
the junction of the two glaciers. In Iowa gulch at this point, as evidenced 
by the moraine material remaining on Printer Boy Hill, the Iowa glacier 
was also about six hundred feet thick and possibly sent a small branch 
some distance into the head of California gulch. The folds have the same 
character as in the previous section, but their crests are farther north. 

Section L. In Section L, which passes through the lower portion of 
Breece Hill and the west slope of Printer Boy Hill, the bed of the Evans 
glacier retains about the same size as in the preceding section, although its 
outlines are somewhat more regular. The Iowa glacier, confined on the 
north by Printer Boy Hill, had spread out somewhat to the south, leaving 
its moraine material well on the crest of Long and Derry Ridge. Califor- 
nia gulch has been cut in the crest of one of the folds mentioned above, and 
in it the lower sheet of Pyritiferous Porphyry is seen to be cutting across 
the formations. 

Section M. Section M, passing through the crest of Yankee Hill and 
just east of Iron Hill, shows the Evans glacier split again into two streams, 
having a total width of over eight thousand feet, but whose thickness is 
MON xu 18 


probably somewhat diminished. The Iowa glacier, on the other hand, seems 
to be contracting as it descends, and in the plane of this section the dis- 
tance between the crests of its bounding moraine ridges is only a little over 
one thousand feet. Here the outline of the Lake beds shows a bay in the 
ancient lake Arkansas and that the older Iowa glacier occupied a wider bed 
than the later one. Except at the foot of the Prospect Mountain, the beds 
lie in an almost horizontal position. 

section N. In Section N the teaching of the Lake beds is still more 
suggestive. The moraine material of the Evans glacier, which was probably 
again united into one sheet, is spread out over a still wider surface; while the 
reconstructed outline of the Arkansas lake shows that from Graham Park 
across to Georgia gulch a ridge then extended, through which the present 
bed of California gulch has been carved out since the Glacial epoch. 

Section o. In Section O, which runs through Fryer and Carbonate 
Hills, only the top of the latter and a portion of what is now California 
gulch probably remained above water during the Glacial epoch. 

Section p. In Section P, which runs across the mesa country, Lake 
beds and Wash cover the whole surface as far as the ridge north of the mouth 
of the Arkansas. The underlying beds are represented as lying in a single 
broad syncline, since, while there may probably be minor undulations, as 
in the sections above, there naturally can be no data for determining their 

As regards underground structure the transverse sections are mainly 
useful as showing probable depths at which the ore-bearing horizon may be 
found. They are too nearly parallel to the direction of major strike, which 
is that of the majority of the folds, to give a correct idea of these folds; and 
their intersection with fault planes, being also at an acute angle, presents a 
somewhat distorted angle of dip. Still it may be observed that on these 
north and south lines the beds have a tendency to form anticlinal and syn- 
clinal folds. Bearing in mind that the prevailing direction of strike is in 
a northwest direction, the continuation of the folds will be found a little 
farther to the north in each successive section; for instance, in Section J 
the -fold under Ball Mountain finds its normal continuation in Section K at 


the mouth of South Evans gulch, and in Section L joins the slight fold at 
the south face of Prospect Mountain. The form of the east-and-west folds, as 
shown in Sections L, M, and N, along the base of Prospect Mountain, sug- 
gests that the mass of Prospect Mountain afforded more resistance to com- 
pression than the adjoining country to the south, so that the folds are com- 
pressed sharply up against it. The reason of this may be found perhaps in 
the unusual thickness of the porphyry bodies on Prospect Mountain, which 
are probably much less plastic than the sedimentary beds. 



In the last two chapters the observations gathered have been pre- 
sented in the form which it was supposed would be most useful to the 
geologist or miner who wished to study the region itself. For those who 
have no occasion to examine the actual ground, it may be well to present 
concisely and in a generalized form some of the more suggestive facts ob- 
served, in a geological rather than topographical order, which will be the 
object of the following pages. 


Archean. That Archean land masses must have existed during the dep- 
osition of the Paleozoic and Mesozoic beds found in this region is abun- 
dantly proved, aside from all structural evidences, by the occurrence at vari- 
ous horizons, in beds evidently of littoral formation, of rolled grains and 
pebbles of Archean rocks. Among these grains and pebbles that which 
would best resist abrasion, quartz, forms naturally the larger proportion, but 
granite and even gneiss are found, and, among the finer materials, feldspar 
and mica often form a large proportion of the sandstones. It is further 
noteworthy that these pebbles do not differ in character from the present 
Archean rocks ; in other words, afford no evidence that the latter have been 
changed by metamorphism since the Cambrian epoch. The fact that only 
.at one point, and this close to a supposed shore line, is any but the charac- 
teristically lowest bed of the Cambrian found in contact with the Archean, 
ehows that the upper surface of the latter, or the bed of the Cambrian ocean, 



must have been comparatively smooth and have presented no abrupt cliffs 
or slopes which were too steep for a uniform deposition of sediment over 

Bedding planes were frequently observed in the Archean in proximity 
to its upper surface, perfectly parallel with and corresponding to the bed- 
ding planes of the Cambrian quartzite immediately above it. As this dis- 
tinctness of bedding planes occurs in granite as well as in gneiss and as in 
general the bedding planes of the Archean, as seen on a large scale, are 
almost invariably discordant with those of the overlying beds, it seems that 
they must have been produced by the pressure of the superincumbent mass 
of beds. 

The eruptive granite of this region is, in all cases, pre-Cambrian in age, 
no instance having been observed of its intrusion into the rocks of any for- 
mation later than the Archean. 

As regards the relative age of the rocks which form the Archean, the 
little study that could be devoted to this subject goes to show that the am- 
phibolites, gneisses, granite-gneisses, and, probably, part of the granites 
proper constituted the older or original formation ; that these were suc- 
ceeded by the distinctly eruptive granites, which cut through and include 
fragments of the above ; and that the vein-like masses of pegmatite are the 
most recent formations of all the Archean rock masses. 

While the structure lines which give evidence of -original bedding or 
stratification in these rocks are less distinctly marked than in other parts of 
the Archean of the Rocky Mountains and were often so obscure that no 
attempts were made to trace out any structural system in the Archean as a 
whole, they are nevertheless sufficiently well marked to suggest an original 
horizontality in the different layers, and that they have been subjected to 
an infinitely greater compression and folding than the later formations, 
while the parallelism of certain upper planes with the lower ones of the 
Cambrian, which has been remarked above, and the varying angle at which 
both are found, show that the Archean has partaken of the folding to which 
the Cambrian and later beds have been subjected. 

Paleozoic. The lower 600 feet of the Paleozoic system in this region, 
comprising the Cambrian, Silurian, and' Lower Carboniferous formations, 


which are remarkably persistent as a whole, though varying from point to 
point in the relative proportions of calcareous and silicious material enter- 
ing into their composition, give evidence, in their even and thin beds and 
in their fineness of grain, of a slow and uniform deposition in quiet and 
rather deep waters. Even in the conglomerate, which is invariably found at 
the base of the series, only very small pebbles of the very hardest and most 
tenacious forms of quartz are found. Neither fragments of Archean -rocks 
nor even feldspar fragments occur in them. The lower calcareous beds also, 
in spite of their dolomitic character, are usually compact and fine grained. 

In the middle member of the Carboniferous, however, a decided change 
in the character of the sediments takes place : they become, as a rule, very 
coarse-grained, cany feldspar and mica and rolled pebbles of granite and 
schist ; they often contain carbonaceous matter, which is sometimes concen- 
trated into actual beds of coal along the borders of the original land mass, 
and remains of plants peculiar to the Carboniferous period are found in them 
at a considerable distance from the supposed shore line. It is evident, there- 
fore, that in the middle Carboniferous epoch the seas became shallower, that 
the abrasion of the land masses was more rapid than theretofore, and that on 
the land vegetation flourished luxuriantly in this mountain region, as it did 
at the same period in other parts of the world. 

During the succeeding Upper Carboniferous epoch and also in the 
Mesozoic era the same coarser character of sediments prevails, although 
carbonaceous deposits are wanting until towards the close of the Creta- 
ceous. Both in the Weber Grits and the Upper Coal Measure formations 
the calcareous deposits are not only very subordinate in quantity but very 
variable ; at one point in a given thickness of rocks only a single thin bed 
of dolomitic limestone will be found, whereas within the same horizon, at 
another point not very far removed, several may occur. 

Dolomitic sediments. One of the most noteworthy facts developed by the 
study of the sediments of this region is the prevalence of dolomites among 
the calcareous deposits. All the calcareous beds below the Robinson lime- 
stone, which was taken as the base of the Upper Coal Measures, are, with the 
unimportant exception of a locally developed silicious limestone ia the Cam- 
brian, true dolomites of varying purity. In the hand specimen they have 


generally the granular structure characteristic of dolomites, and under the 
microscope it is seen that there is little or none of the twin structure pecul- 
iar to calcite, and that they are therefore composed, not of a mixture of 
calcite and carbonate of magnesia, but of true dolomite or double carbon- 
ate of lime and magnesia. The upper bed of the Robinson limestone, on 
the other hand, and also the few limestones of the Upper Coal Measure 
formation that were examined are true limestones and have a characteris- 
tically different appearance from the dolomites in the hand specimen. They 
are fine grained and compact, instead of granular, generally of light color, 
and often have the conchoidal fracture and fine texture of a lithographic 
stone. The lime and magnesia contents of twenty different specimens 
of limestones from different horizons and localities are given in Table VI, 
Appendix B. 

It is also noteworthy that all of these limestones, as far as tested, were 
found to contain chlorine in appreciable amount. Microscopical examination 
of the Blue Limestone collected at Leadville, whose contents in chlorine 
amounted to one-tenth of one per cent, showed that it probably occurs in 
the form of a solution of chloride of sodium, in extremely minute fluid 
inclusions within the grains. 

These investigations were made in the hope that they might throw 
some light upon the cause and manner of formation of dolomites in gen- 
eral. It can only be said that it seems evident that the magnesia is an 
original constituent of the rocks, and not introduced later by metamorphic 
action. It were difficult to conceive of such an action, for instance, in the 
case of the Robinson Limestone, the upper fifteen to twenty feet of which 
are almost chemically pure carbonate of lime, while the lower ten feet con- 
tain less than 88 per cent, of carbonate of lime, the rest being carbonate of 
magnesia and insoluble material ; or how such metamorphic action should 
be so widespread and uniform over this great area and yet stop at a given 
bed or horizon. 

T. Sterry Hunt, 1 who, with others, has advocated the theory that dolo- 
mites are formed by the actual precipitation of the carbonate of magnesia, 
maintains that its separation requires the absence of chloride of calcium 

1 Chemical and Geological Essays, Bostou, 1875, p. 92. 


from the waters in which it is deposited and that isolated or evaporating 
basins are indispensable conditions of the formation of dolomite. In this 
particular region these conditions might have been fulfilled, since the 
Archean land masses certainly inclosed the sea on two sides. His theory 
requires, however, that all the lime contained in the sea waters should have 
first been precipitated by the carbonate of soda, which would then act on 
the chloride of magnesium and throw it down as carbonate. It would 
seem, however, from the character of the rocks, which are formed of crys- 
talline grains of the double carbonate, that the two salts were probably 
precipitated at the same time and that a certain amount of chloride in solu- 
tion was inclosed in the grains as they crystallized. 

As regards the question whether carbonate of lime is more -readily dis- 
solved out of a dolomite than carbonate of magnesia, the evidence goes to 
show that percolating waters act upon the double salt, and not upon its 
more soluble member alone, since the veins and cavities, such as are shown 
in the lower specimen on Plate VI (p. 64), which have been refilled by white 
crystalline material deposited by these waters, are found to have the same 
composition as the original dolomite. Moreover, where the entire rock has 
been apparently changed by the action of waters, as in the so-called "lime 
sand" found in the mines, which is Blue Limestone from which the cement- 
ing material of the grains has been removed, or in the case of a given 
bed in the White Limestone of Dyer Mountain, which in one part has, by 
this action, from a compact light blue rock, become clayey in structure and 
pink in color, analysis shows that the proportions of carbonate of lime 
and magnesia remain essentially unchanged, whatever variation there may 
have been in the other constituents. 1 

In both the above cases the metamorphism, or change in the character 
of the limestone, must have taken place about the time of the deposition of 
the ore bodies in these regions, and would therefore not have been pro- 
duced by surface waters. The action of surface waters, using this term in 
its ordinary, restricted sense of waters which come from the surface under 
essentially the same conditions that exist at the present day, is apparently 
different from the above, judging from the following observation. 

1 Sec Analyses 5, 6, 9, and 10. Table VI, Appendix B. 


The conglomerates which form the Lake-bed deposits are often found 
to have a calcareous cement, that can be readily separated from the pebbles 
which it incloses. This conglomerate is of so recent date that at the time 
of its formation the structural conditions of the range must have been essen- 
tially those which prevail at the present day, and the waters from which the 
cementing material was derived were surface waters, which may be sup- 
posed to have drawn their calcareous constituents from the outcrops of the 
various dolomitic beds of the lower Paleozoic series. Chemical tests show 
that the cement is made up almost entirely of carbonate of lime, with little 
or no carbonate of magnesia. It would seem, therefore, that when exposed 
to the action of surface waters the dolomites of this region have yielded up 
their carbonate of lime more readily than their carbonate of magnesia. This 
may be due to a previous disintegration under the action of atmospheric 
agents which rendered them more attackable or to a superior solvent power 
' of surface waters over underground waters in their action upon the carbonate 
of lime. 

Serpentine. The development of serpentine in the Silurian beds of this 
region is, it is believed, the first observed instance of its occurrence in the 
Rocky Mountain region, and therefore deserves some detailed mention. Its 
principal point of development is in the Red amphitheater in Buckskin gulch, 
on the south face of Mount Bross, where it is found mainly in the transition 
beds at the base of the Silurian formation, though extending to a limited 
extent up as far as the base of the Carboniferous. It was also observed in 
limited development on the cliffs at the south base of Mount Lincoln, and 
specimens were obtained in the Leadville district from the Comstock tunnel 
in California gulch, where its exact horizon could not be definitely deter- 
mined. No actual serpentine was found at any other point, but a greenish- 
colored bed was observed frequently at about the same horizon, which 
by a microscopical examination of certain specimens was proved to contain 
amphibole or pyroxene. 

As developed in the Red amphitheater, it occurs generally in limestone, 
forming a greenish, veined, and clouded rock, like verd-antique, the veins 
or streaks of serpentine generally running parallel with the stratification, 
but sometimes crossing it at right angles. It also occurs in a yellow, homo- 


geneous-looking rock, resembling yellow beeswax, which proves by analysis 
to be an intimate mixture of calcite and serpentine. A complete analysis 
of the soft green material from a specimen of the darker-colored rock is given 
in Analysis I, Table VII, Appendix B, which proves it to be an almost normal 
serpentine, the oxygen ratio being 3 : 3.95 : 2.11, instead of 3:4:2, which is 
the theoretical proportion. Analysis II, in the same table, is that of the. whole 
mass of yellow rock, which is found to contain 57.57 per cent, of carbonate 
of lime. If this be deducted, the composition of the residue is essentially' 
the same as that of I. The microscope confirms the conclusion that the 
rock is a simple mixture of serpentine and calcite, as no other mineral can 
be distinguished by it. It also shows that the major part of the rock is 
in grains which show the cleavage distinctly, whereas the small grains of 
calcite which are sometimes found in the dolomites show no such cleavage 
lines ; hence it is evident that the calcite has been recrystallized. 

origin of the serpentine. It is evident, from the manner of its occurrence 
in and intimate admixture with the sedimentary rocks, that the serpentine 
is not of eruptive origin. It seems equally improbable, from its extremely 
local development, that it could have been formed at the time of the pre- 
cipitation and deposition of the original sediments. It is noteworthy, 
further, that the localities where it was found have been near centers of 
eruptive action and of consequent intense metamorphism. The dolomites 
of this horizon, which are all more or less silicious, contain all the constit- 
uent elements of serpentine, except water. If by the addition of this ele- 
ment a reaction between it and the silica and magnesia could be brought 
about, serpentine might have been formed directly from the dolomites. As, 
however, it is difficult to conceive of such a direct reaction, it seems better 
to seek some intermediate step. Among the specimens of Serpentinous rock 
from the Red amphitheater, one has gray portions, comparatively free from 
serpentine, in which fibrous silky crystals can be observed. The microscope 
showed that these are amphibole crystals, and, further, that pyroxene was 
present. Analysis III, Table VII, shows the composition of these silky 
crystals after they had been separated from the rest of the mass, which is 
practically that of actinolite. The part supposed to be pyroxene, which is 
distinguishable as being less lustrous, was not analyzed. Now, the for- 


mation of serpentine as an alteration product of amphibole and pyroxene has 
been not unfrequently observed and actual pseudomorphs have been found. 1 
It thus appears evident that a part at least of the serpentine in these 
rocks is an alteration product of amphibole and pyroxene, and in further 
confirmation of this hypothesis the microscope shows, in specimens of a 
green silicious rock from the lower part of the Red amphitheater and of a 
similar rock from the south base of Mount Lincoln, among fresh and un- 
mistakable amphibole crystals, some in process of decomposition, whose end 
product is serpentine, the remaining components of the rock being quartz 
grains and calcite in alternate layers. 

J. D. Dana, 2 in treating of similar occurrences of serpentine in the dolo- 
initic limestones of Southern New York, supposes that the process of change 
was that by a first metamorphism the uncrystallized dolomite became pene- 
trated with tremolite, actinolite, and other magnesian silicates, and that 
"these beds underwent a later transformation, converting the tremolite and 
other magnesian silicates and part of the remaining dolomite into hydrous 
magnesian silicates and mostly into serpentine." At first glance the Mosquito 
Range phenomena seem to present a further analogy with those of Southern 
New York in that there are presented two periods of possible metamorphism 
(or activity of metamorphic action), viz, that following the intrusion of the 
porphyries and diorites and that following the folding and faulting which 
accompanied the uplift of the range. There is, however, no evidence that 
the dynamic movement was either accompanied or directly followed by any 
widespread metamorphic action. The decomposition of metallic minerals, 
which was a metamorphic action, preceded this movement and followed the 
eruption of porphyries. 

As to whether the serpentine has been derived entirely from amphibole 
and pyroxene, or whether a part may have been derived directly from 
dolomite, as suggested by Dana in regard to the New York occurrence, 
no definitely conclusive evidence has been obtained. No opportunity was 
offered for tracing the yellow rock, which would seem probably to have 

1 J. Roth: Allgern. u. chein. Geologic, pp. 123, 127, 131. Berlin, 1879. A. Lagorio: Mic. Anal. 
Ostbaltischer Gebirgsarten, p. 43. E. B. Hare : "Die Serpeutiu-Masse von Reichensteiu" (Neues Jalir- 
bucb, II. Bd., p. 346. 1880.) 

^American Journal of Science, Vol. XX, p. 32. July, 1880. 


been derived from a limestone bed relatively free from quartz, to a less com- 
pletely altered condition, where it might have been seen whether there had 
been a previous formation of amphibole. In the dark-green rock, however* 
there seems little doubt that the serpentines 'are derived from silicates. 

With regard to the formation of amphibole and pyroxene, their distri- 
bution seems wider and more even, and the question presents itself whether 
they have been formed in situ by a slow process of metamorphism pre- 
ceding the appearance of the eruptive rocks, or after this period and imme- 
diately preceding that of the serpentine, or, again, whether they are simply 
derived from the Archean rocks mechanically. In favor of the last sup- 
position is the fact, observed by Mr. Cross in one specimen, that the amphi- 
bole penetrates the quartz grains and is sometimes entirely encircled in 
them, and that these latter contain fluid inclusions with moving bubble. 


The most striking features in the geological structure of this region 
are the forms of the folds and the close relation between them and the 
great faults which traverse it from north to south. 

Folds and faults. The typical form of the former is what has been called 
the S-fold, in which the anticline has a steep and almost vertical face to 
the west, or towards the original land mass of the Sawatch, and a gentle 
slope to the east, while in the adjoining syncline the conditions are re- 
versed, and the gently rising slope is to the west. This is the most natural 
form of fold which would result from the supposed cause of uplift of the 
range, namely, a horizontal thrust of the beds against the Archean mass of 
the Sawatch. In a fold produced in this way the line of greatest tension, 
and where the tendency to fracturing and displacement would be greatest, 
is, as shown by DaubreVs well-known experiments, 1 along this steep side 
of the fold, and in point of fact it was found that along this line occur the 
great strike-faults of the range. 

By reference to the sheets of sections (Atlas Sheets VIII and IX) it will 
be seen that the great Mosquito fault, which extends for an unknown dis- 
tance be} 7 ond the northern limits of the map, and its two southern branches, 
the London and Weston faults, fulfill in the main these theoretical conditions. 

'A. Daubr^e : Geologic Experimental, p. 321. Paris, 1879. 


It is rarely possible to trace upon the surface the actual line of a fault 
or the structure lines of the immediately adjoining beds, for the reason that 
the rocks are generally metamorphosed and disintegrated to such an extent 
as to render them obscure. The theoretical studies of fault structure have, 
moreover, been mainly made in underground workings, especially in coal 
mines, where it is often the case that the movement of displacement is so 
slight and the thickness of beds involved so small that it is questionable 
whether they should not more properly be considered as joints, rather than 
as fulfilling the same conditions as these great faults many miles in length 
and with displacements involving thicknesses of beds of as many thousand 
feet. Even in this region, where the opportunities for observation are excep- 
tionally favorable, the actual fault planes and the structure lines of the ad- 
joining beds can but rarely be distinguished. Either only Archean rocks, 
in which no structure lines are visible, are to be found on one side of the 
fault, or the surface conditions are such that the structure lines are entirely 
obscured in its vicinity. In drawing the sections, moreover, the endeavor 
was to represent the facts as far as observed, without reference to any struct- 
ural theory, and they were already engraved before any theoretical study 
of the structure as a whole was undertaken. If, then, in any case they 
misrepresent facts, the error is as likely to be against, the above theory as 
in its favor. 

At the northern edge of the map a syncline is plainly traceable in close 
contact with the fault line on the west of the Mosquito fault, and the remains 
of the corresponding anticline on its east side are found in the fragment of 
Cambrian quartzite resting on the Archean just beyond the limits of the map. 
From here southward to Empire gulch either the Archean alone adjoins the 
fault line or the stratification lines of the sedimentary beds in its immediate 
vicinity are entirely obscured ; those given in the sections are only the the- 
oretical prolongation of dips observed at such a distance that there is room 
for a very marked flexing to have occurred before they reached the fault 
plane. On Empire Hill is what might be classed as a monocline to the west 
of the fault, were it not that its continuation farther south at Weston's pass 
shows that it is part of a deep syncline, cut off by the fault, and a portion of 


the crest of the corresponding anticline on the east side still caps Weston's 
Peak. It is in the London fault, however, that the relations of the fold and 
the fault are most clearly seen, because the sedimentary beds still remain on 
either side to show the structure lines and erosion has cut down into the 
rock mass so deeply as to afford to the observer actual sections of the earth's 
crust several miles in length and one to two thousand feet in thickness. 
These have been described in detail on pages 143-165 and illustrated by 
sketches in Plates XV, XVI, and XVII, so that it will be hardly worth 
while to redescribe all the conditions here. 

It is probable that the steepness of the angle of dip of the beds on 
either side of the fault plane in these cases may be due to a continu- 
ation of the movement of contraction, or the lateral thrust, since the original 
faulting and folding, for it is now generally conceded by geologists that the 
elevation of mountains is continued in a somewhat modified form long after 
the original dynamic movement, and may very probably be going on at the 
present day. In the case of the fold at Weston's pass a lateral movement 
along the fault plane seems also necessary to explain the observed condi- 

This dipping downward of the beds on either side of the fault would 
seem at first sight to be an exception to what is given in text-books as 
the rule for the plication of beds adjoining a fault plane, namely, that they 
bend in opposite directions down toward the fault on one side and up 
toward it on the other. It is not really so, however, as mature reflection 
will show. In the case presented by the text-books of strata dipping in 
opposite directions on either side of the fault, if the beds were brought back 
to the position they occupied before the displacement, they would be 
found to have a simple monoclinal fold, such as is described as common in 
the Colorado Plateau region by the geologists who have written upon it, 
and which, according to them, is often associated with a fault. These folds 
and faults differ from those in the greater intensity of the plication and in the 
different position of the fault plane in regard to the flexure. If one of the 
S-folds described here could be drawn back to its incipient state of flexure 
and the strata adjoining it brought to an approximately horizontal position, 
it would gradually become the monoclinal flexure described by them; or 


one might imagine the monoclinal flexure under conditions of greater press- 
ure, and with a general uptilting of the whole sedimentary series involved, 
developing into one of these S-folds. As regards the position of the fault 
plane, in the supposed case of the monocline it actually cuts the steep side; 
but here it cuts generally through the syncline on one side of it. It can 
readily be seen by reference to the section that a comparatively slight lat- 
eral displacement of the fault planes to one side or the other would produce 
the above-quoted conditions of an opposite dip on either side of the fault, 
or, to be more accurate, opposite as regards the fault plane, since the actual 
dip is the same on both sides of the fault in the case of the monoclinal fault 
and reversed in the case described here. 

In connection with the shorter and less important faults which trav- 
erse the region of the Leadville map, the folds are much more gentle and 
less strongly marked than in the case of these larger faults ; but in almost 
every case where it is possible to obtain data it is found that the same inter- 
dependence of folding and faulting exists. 

Hade of faults. In the few instances where it was possible to obtain act- 
ual measurements of the hade of the fault planes, or their inclination from 
the vertical, it was found to be towards the downthrow side, or that the 
plane of the fault slopes away from the side which has risen ; this is the 
condition which generally prevails, and it is explained on the theory that the 
uplifted side has thus a broader base than the downthrow side. In only a 
few isolated cases was evidence found, and only indirect evidence at that, 
of the opposite conditions, or of a reversed fault. The angle of hade 
in the observed cases was almost equal to the angle of dip of the strata'; 
in other words, the fracture was directly across the beds. In drawing the 
faults where the angle could not be observed, as was the case in the major- 
ity of instances, they were constructed to accord with this condition. The 
objection has been made to the assumption that the normal hade of faults 
should be in the direction of downthrow, that it is opposed to the theory 
that faulting, like folding, is the result of contraction, inasmuch as hading in 
this direction tends to lengthen the linear space occupied by a series of beds 
on a given cross-section, rather than to contract it. This may be graphically 
seen in the sections on Atlas Sheet VIII. In Section A, for instance, where 


only one fault crosses the section, the linear contraction of a given bed, as 
there drawn, is about three thousand feet in the length of the section, or 
3 per cent On Section D the apparent amount of contraction is the same, 
although the beds are much more sharply flexed ; but it is found that, by 
reason of the angle of hade given to the faults, there has been 1,500 feet 
of apparent expansion of the beds; or, if the fault planes had been made 
vertical, the same amount of flexing would have given 1,500 feet more 
length to the beds and the contraction would have been 4,500 feet, or 5J 
per cent. The sections present probably an exaggerated statement of what 
actually exists, for it is possible and even probable that the planes of the 
great faults stand more nearly in a vertical position ; still, observation ren- 
ders it probable that the average hade in the faults of this range is with 
the downthrow, and for this reason the displacement of the faults has not 
tended to contract the linear distance occupied by a given series of forma- 
tions on a transverse line, but rather to expand it slightly. It seems proba- 
ble that the plication of the beds has been a gradual and uniform move- 
ment, though relatively accelerated at the period assigned to the dynamic 
movements; but that the actual fracturing of the beds along the present 
fault planes was primarily produced by some violent shock, similar to the 
earthquake shocks of the present day; that the direction of a fracture 
plane across the beds, as thus primarily determined, would not necessarily 
be dependent on the force of contraction, although its position would 
naturally be on lines of greatest tension or weakness. 

It may also be conceived in a region like the one under consideration 
that, while the folding is evidently a result of tangential contraction, the 
faulting may be, in part at least, the result of radial contraction. It is 
probable that tangential pressure acts only on a comparatively thin shell of 
the upper crust of the earth, for very sharp folds, where observation in 
depth is possible, are found to become gradually more rounded and gentle 
as the distance from the surface increases ; also, that the force which has 
been exerted in an intensely plicated region is the expression of the accu- 
mulated energy of contraction over a wide area. Thus, in the case of the 
Mosquito range, tangential pressure may be conceived to have pushed up a 
roll of the earth's crust into a ridge, which would have been much higher 


than the restoration of the eroded beds in their present position would give, 
if it had not been for the counteracting effect of faulting ; and faulting 
might, in this sense, be considered a result of subsidence or of radial con- 
traction. It is easily seen, for instance, by studying any one of the given 
cross-sections of the Mosquito range, that were the movements of the faults 
reversed, so as to bring the beds on either side of each back into their orig- 
inal position, and thus leave them as they would have been if influenced 
by plication alone, the range would have been about four thousand feet 
higher than at present, supposing erosion to have acted under those condi- 
tions with the same energy that it has under the present. This view of the 
elevation of the range involves, it is true, a subsidence of the region ad- 
joining the Sawatch shore line and probably of the whole Sawatch mass. 
Subsidence and elevation in cases like this, which refer to a far distant 
period of the earth's history and where limited areas are involved, are more 
or less interchangeable terms, since the only fixed point to which they can 
be related is the .center of the earth, whose distance cannot be determined 
with a possible error less than the amount of movement involved, and we 
have to content ourselves with the assumption that there must be a tend- 
ency in all movements of the earth's crust to preserve a certain equilibrium, 
and that, where one portion of the crust has been elevated in relation to an 
adjoining one, the apparent movement is probably the sum of an actual 
elevatory movement on the one side and of a subsiding movement on the 
other, each of which is necessarily less in amount than the apparent move- 

The same may be said of areas large enough to assume an almost con- 
tinental importance. Thus the Plateau region of the Colorado River has 
evidently subsided relatively to the adjoining mountain areas of the Wa- 
satch and of the Rocky Mountains, as shown by the great average differ- 
ence of level of corresponding formations in these areas; but the Plateau 
region must always have been relatively lower than these, since it was 
from the abrasion of their land masses that its sediments were in large 
measure derived. It may, however, be considered in general to represent 
an area of subsidence, and the others to be areas of elevation; and the type 
structure which prevails there, namely, that of broad level blocks descend- 



ing abruptly along given lines by monoclimil flexures and faults to lower 
levels, to be more frequently the result of subsidence, while the movements 
in the adjoining mountain masses were probably more often true move- 
ments of elevation. 

The one-sided or s-shaped fold. In the above remarks considerable stress has 
been laid upon the one-sided or S-fold and its frequently associated faulting, 
because it seems to be the extreme development of the most common form 
of plication throughout the Rocky Mountains and the region of the Great 
Basin. In the latter region it often happens that only one side of the fold 
protrudes above the Quaternary deposits, which cover the greater portion of 
its surface, so that the narrow mountain ridges present only a monoclinal 
slope. For this reason the structure of what is called the Basin province 
has been characterized as a region of faulted blocks uptilted in different 
directions and practically without plication. 

I dissent from this reading of the geological structure, first, because 
my own observations in the region mentioned have shown many unmistak- 
able instances of the above-mentioned structure, in which it is true the 
flexing is often gentle, but nevertheless a true plication, and which have led 
me to believe by analogy that in other cases, could the structure beneath 
the valleys be seen, the missing faulted-down members of the fold would 
be found ; secondly, because the diversely tilted blocks which are given in 
the sections involve what seems to me to be a geological impossibility, or at 
least one which is not yet found possible by observation, namely, the actual 
annihilation of considerable wedge-shaped segments of stratified beds by 
the simple action of faulting. Even in the Uinta Range, which differs from 
the Rocky Mountain ranges in that it is the truncation of a complete arch 
of sedimentary strata, with no evident pre-existing elevation beneath it, the 
anticlinal fold has the one-sided structure. The axis of this range is along 
the northern edge of the uplift, to the south of the axis the beds descend 
in gentle slopes ; to the north they dip steeply at angles of 40 or 50, and 
are partly faulted along this steep side. On a line with this steep side, at 
the eastern end of the range, is a submerged ridge of Archean, whose resist- 
ance, as I have already suggested, 1 probably caused the sharper and more 

"Exploration of the Fortieth Parallel, Vol. II, Dcscr. Geol., p. 201. Washington, 1877. 


complete plication of the formations at that end of the range. This ridge 
may extend westward at a greater depth along the whole length of the 
range, and be the unyielding mass whose resistance caused the sharper flex- 
ure along its northern edge. 

I also differ from the views advanced by some geologists with regard 
to the structure of the Rocky Mountain region, or the Park province, as 
they designate it. One considers the type structure of this region, as repre- 
sented by the Colorado and Park Ranges, to be the same as that of the Uinta, 
viz, that the sedimentary strata formerly arched over them, and that the 
uplift was that of a broad platform raised by vertical movement, having a 
fault or monoclinal fold along either edge. Another, in speaking of the 
whole Cordilleran system east of the Sierra Nevada, says it has no plication 
properly speaking, as expressed by the folds of the Appalachian, although 
he admits that some regions show a certain amount of flexure, including in 
them probably the Basin and Park provinces. In regard to these provinces 
he says that these flexures are not, so far as can be discerned, associated with 
the building of the existing mountains in such a manner as to justify the 
inference that the flexing and the rearing of the ranges are correlatively 
associated ; that the flexures were in the main older than the mountains, and 
that the mountains were blocked out by faults from a platform which had 1 
been plicated long before, and after the irregularities due to such pre-ex- 
isting flexures had been nearly obliterated by erosion. He says further that 
the amount of bending caused by the uplifting of the ranges is just enough 
to give the range its general profile and seldom anything more. 

I have already shown, in Chapter II, my reasons for considering that 
the main Archean masses of the Rocky Mountains, as represented by the 
Colorado and Park Ranges, were never submerged, and that therefore the 
sedimentary strata could not have arched over them as they did over the 
Uinta Range; also, that the Mosquito Range, which might be considered 
at first sight an exception, is not geologically part of the Park Range, but an 
uplift of later formation, contemporaneous with and of analogous formation to 
the so-called " Hog-back" ridges on the east flanks of the Rocky Mountains, 
though of more complicated structure, owing partly to its eruptive masses and 


partly to a more intense movement of compression. On the eastern flanks of 
the mountains the anticline, east of the first monoclinal slope which rests 
directly on the Archean, is rarely seen, being concealed beneath later 
deposits. Its slopes are probably relatively gentle, as it is not compressed 
between two Archean masses like those of the Mosquito Range, but has a 
broad mountainless area at its back. Could this fold be seen it would 
probably be found to have the S character ; that is, a steeper slope to the 
west. The fact that the monoclinal slopes on the eastern flanks are some- 
times very steep I judge to be due to a later movement of contraction since 
the dynamic movement, by which the upper beds have been pushed against 
the Archean and the beds which rest directly on it, and thus brought into 
a vertical or even an inverted position. 

As regards the correlation of folding and faulting, the geological evi- 
dence, as I read it, is entirely opposed to the idea that the uplift of the 
ranges was independent of and later than the flexing, and produced mainly 
by faulting. The evidence of the Mosquito Range, which may be fairly 
taken as a type, though perhaps an extreme one, of Rocky Mountain struct- 
ure, certainly shows a close interdependence between folding and faulting. 
That the rocks forming the Archean land masses were plicated and eroded 
long before is quite evident, but that the}' were blocked out by faults and 
lifted into a platform is purely hypothetical and incapable of proof. Again, 
while the closely appressed folds of the Appalachians are rarely found in the 
Rocky Mountains, I consider the folding that does exist there none the less 
a true plication. The peculiarly regular, narrow folds of the Jura Mount- 
ains and of the Appalachian system are simply the extreme type of closely 
folded strata, due to peculiarly favorable conditions which it is not now 
worth while to discuss at length, and differ from those of the Rocky Mount- 
ains in amount rather than in kind. 


The eruptive rocks of this region fall naturally into two groups or 
series, whether considered from the point of view of their age, of their man- 
ner of eruption, or of their internal structure and composition. 


Here, as in all attempts at establishing geological classification, there 
are found to be occurrences which form intermediate or transition 
members between the two groups, but yet do not invalidate the legitimacy 
or advisability of establishing such a division or classification. Geologists 
have hitherto been divided in opinion as to the nature of the relation be- 
tween the age of an eruptive rock and its internal structure and compo- 
sition, the extremists on one side maintaining that this relation is an abso- 
lute and fixed one, and that where, as is so often the case, its geological 
and external structural relations furnish no evidence as to the age of a rock, 
a careful study of its internal structure is sufficient to determine within 


certain limits its period of eruption; those of the opposite school maintain 
that no such relation exists, that the correspondences observed are merely 
accidental coincidences not dependent on age, and that the many sub- 
divisions established by the former school are not legitimate, and, inasmuch 
as there are an infinity of intermediate members, could with advantage be 
reduced to a few general divisions. The manner of eruption, whether as 
an intrusion or as a surface flow, has not in general been considered an 
essential function of classification. In this region it would seem that the 
characteristic differences of internal structure of the above two classes de- 
pend rather upon the conditions under which they have consolidated than 
upon the absolute geological age of either class, although their relative ages, 
which are distinctly marked, correspond, as it happens, with the differing 
conditions of consolidation. 

Age. As regards their period of eruption, the rocks of this region may 
be divided into an older and a younger series, the former of which were 
erupted before the dynamic movement which caused the uplift of the range 
and were involved with the inclosing sedimentary strata in the consequent 
folding and faulting, while the latter are of later date than that dynamic 

An exact definition of the age of either group is unfortunately not yet 
possible. In the first place, the time of the dynamic movement is assumed as 
at the close of the Cretaceous period, this assumption having been adopted 
by the consensus of the geologists who have studied the Rocky Mountain 
region, for the reason that the Tertiary beds, where found in contact, are 


seen to have been deposited unconformahly upon the Cretaceous. But in 
the district included in this examination no Tertiary beds are found. More- 
over, it is not impossible that later and more detailed studies may lead to a 
modification of this view. Secondly, although the older eruptives are only 
found in Paleozoic formations within the limits of the map, rocks almost 
identical were observed in Triassic and even in Cretaceous strata, not far be- 
yond those limits. Moreover, the same class of rock, as will be shown below, 
is found in other parts of Colorado to cut through the latest Cretaceous beds. 
It seems probable, therefore, that though the eruption of this type of 
rock may have commenced much earlier, it lasted in this region till near the 
close of the Cretaceous. As regards the age of the younger series; the en- 
tire absence of Tertiary beds in the region renders it impossible to assign their 
eruption to any particular division of this era. The time that elapsed between 
the eruption of the last of the older series and the first of the younger must, 
however, have been much longer than the above statement would seem at 
first glance to wan-ant, since in it not only were the inclosing beds elevated 
above the ocean, and by plication and faulting brought practically into their 
present position, but erosion must have removed their upper portions down 
to a general level, which could not have been much higher than that of the 
average peaks and ridges of the present day. 

But little direct evidence was obtained as to the relative age of the 
varieties composing either group, but what was found, as well as the indi- 
rect evidence and a certain indefinable habitus of the rocks, goes to con- 
firm Clarence King's theory 1 that in each series of rocks composing a local 
eruption, and which may be considered in general to have a common source, 
the acidic rocks were the earlier, and the more basic followed in the order of 
their relative basicity. Thus, among the older series of rocks the more basic 
porphyrite is younger than the acid quartz-porphyries. Direct evidence of 
this is confined to the single instance observed of actual contact of the two 
varieties of rock on the extremity of the east spur of Mount Lincoln; and 
here, owing to the character of the exposure, the apparent cutting of Lin- 
coln Porphyry by porphyrite cannot be considered entirely unquestionable. 
The external habit and internal structure of the rocks, however, both con- 

1 Exploration of the Fortieth Parallel, Vol. I, Systematic Geology, p. 715. Washington 



lirui the above conclusions. In the hand specimen some of the porphyrites 
might readily be taken for Tertiary eruptives. Among quarti- porphyries 
the White Porphyry, which is the most acid of the group, not only has the 
characteristics of an older rock in its internal structure, but is actually cut 
bv transverse bodies or dikes of the Gray or Lincoln Porphyry, and a small 
sheet of it, together with inclosing sandstones, is included in the great mass 
of Sacramento Porphyry. An apparent exception to this evidence of the 
earlier age of its principal mass is found in the existence of two dikes of 
White Porphyry cutting Lincoln Porphyry, on the north wall of Cameron 
amphitheater, but their mass is relatively very small and the occurrence 
altogether an exceptional one. 

Among the Tertiary eruptives Nevadite seems to be older than the 
andesites, judged by its internal structure and its geological surroundings, 
but the rocks are so widely separated that no direct evidence was obtain- 

Manner of occurrence. The two groups are further distinguished by the 
fact that the older rocks are entirely intrusive and the younger extrusive ; 
in other words, that the former never reached the surface, but were consoli- 
dated within the sedimentary strata and under the pressure of a considerable 
mass of overlying rocks, while the latter were, as far as can be determined 
at the present day, actually extruded upon the surface before final consoli- 
dation. This is an important distinction in its bearings upon the internal 
and petrographical structure of the rocks, and one upon which it seems geolo- 
gists have hitherto not laid sufficient stress. 

intrusive sheets. The greater mass of the older rocks occurs as sheets 
between the strata of sedimentary rocks, generally following a given hori- 
zon over great distances. That they were not poured out upon the surface 
and the overlying sedimentary beds deposited upon them that is, that they 
are not interbedded sheets is abundantly proved by the facts that they fre- 
quently cross the strata from one bedding plane to another and that they 
also occur as dikes cutting across the strata transversely, whose actual con- 
nection with the intrusive sheet it was sometimes possible to observe; large 
fragments of the overlying beds are, moreover, often found entirely included 
in them. The great number and extent of these intrusive sheets are very 


remarkable. They vary in thickness from a foot or two up to over a thou- 
sand feet. In Mosquito gulch sheets of porphyrite averaging 20 feet in 
thickness can be traced continuously on the canon walls for several miles 
without showing any vent, and the sheet of White Porphyry which covers 
the Blue Limestone is shown by its outcrops to have been practically con- 
tinuous over the area of the southern half of the Mosquito map. The only 
direct evidence of a channel or vent leading to this sheet of porphyry from 
below is at White Ridge, the point where it occurs in maximum thickness, 
whence it might be assumed to have spread out from this point as a center 
of eruption. In this case it would have spread ten miles from its center, 
gradually thinning out from 1,500 feet over the vent and 500 feet within a 
mile or two of it to 20 feet at the farthest point observed. The recon- 
structed form of this body, as shown in Section F, corresponds to that of 
the dome-shaped bodies in the Henry Mountains, described by G. K. Gil- 
bert 1 under the name of laccolites. Indeed, it is evident that the manner 
of eruption of all the older igneous rocks of this region was analogous to 
that of the Henry Mountain rocks, although the amount of plication, dis- 
location, and subsequent erosion to which these have been subjected ren- 
ders it more difficult to reconstruct accurately their original form, and it is 
probable that if they were restored they would be found to want the regu- 
lar, symmetrical shapes he describes. The large dome-shaped bodies are 
rare, but relatively thick sheets, one above another, are often very numer- 
ous ; for instance, in the Ten-Mile district, just beyond the northern limits 
of the map, the outcrops of seventeen were observed in a single transverse 
section across an estimated thickness of less than 15,000 feet of sediment- 
ary strata. 

Dikes. Normal dikes in the sedimentary strata were rarely observed, 
and wherever the sheets cross the strata transversely it is usually at a very 
low angle. In the Archean formations, on the other hand, the older rocks 
were almost invariably found in the form of narrow and rather irregular 
dikes, as a rule not over fifty feet in thickness, the principal eruptions being 
two large bodies of diorite, whose outlines were not very accurately deter- 

1 Geology of the Henry Mountains Washington, 1877. 


The Archean exposures studied in this region were once covered by 
portions of the same sedimentary series in which these intrusive sheets 
are now found. It may, therefore, be assumed that the form of the chan- 
nels through which the fused masses were forced up into the overlying 
beds is fairly represented by the average outline of these dikes. According 
to this reasoning it is evident that the channels in the Archean were ex- 
tremely small as compared with the extent of the sheets themselves, and were 
rather in the form of the narrow fissure, which has been supposed to be the 
source of so-called massive eruptions, than of the rounded "necks," which 
have sometimes been observed as the actual vents of volcanic eruptions. 

Relation of form to composition. In comparing the relative form of the in- 
trusive bodies with the composition and structure of the rocks which com- 
pose them, it is found that the more basic the rock the thinner is the sheet 
and the greater its relative extent in a horizontal direction. It is true 
the range of relative acidity in this region is not the very widest, the White 
Porphyry, whose beds are relatively the thickest, having about 70 per cent, 
of silica, while the hornblende-porphyrite, which occurs in the thinnest sheets 
and at the same time with relatively great horizontal extension, has a little 
over 56 per cent, of silica. Basalts range from 45 to 50 per cent, of silica. 
. The great sheet of White Porphyry has an estimated thickness of 1,500 feet at 
the point of its supposed intrusion from below, and the least thickness ob- 
served is 20 feet. The Sacramento Porphyry, which is slightly less acid 
(having 65 per cent, of silica) and whose greatest thickness is found im- 
mediately adjoining the White Porphyry laccolite, is evidently somewhat 
thinner, being probably less than 1,000 feet at its maximum, while the horn- 
blende-porphyrite sheet of Mosquito and Buckskin gulches is found in a 
maximum thickness of 25 feet, and frequently thins to 6 feet, or even less; 
yet its practical continuity over considerable areas is even more readily 
evident than in the case of the larger masses of quartz-porphyry. A more 
remarkable instance of the great relative area of a basic intrusive sheet is 
that of the Whin Sill, in Northumberland, England, which, according to 
Messrs. Topley and Lebour, 1 is a basaltic intrusive sheet, that has been traced 
with unimportant breaks for a distance of 75 to 80 miles in a thickness of 

'Quarterly Journal of tho Geological Society, XXXIII, pp. 406-421, 1877. 


about seventy feet. It occurs in the Carboniferous formation, and, like the 
porphyrite of Mosquito gulch, while apparently following a given bedding 
plane, actually changes from one horizon to another within a vertical range 
of about seventeen hundred feet. 

It is apparent from the above facts that in underground flows of igneous 
rocks, as in lavas flowing on the surface, the coefficient of extent in relation 
to thickness of flow is a function of the relative basicity and consequent 
fluidity of the fused mass. 

Amount of intrusive force. In studying these intrusive sheets one is forci- 
bly impressed with the magnitude of the force exerted during the intrusion 
of the lava, which here seems almost capable of actual measurement. 
Assuming as a type of these sheets the great body of White Porphyry 
above the Blue Limestone, it is seen that at its thickest point under 
White Ridge it has pried open the strata a distance of about fifteen 
hundred feet vertically, since that estimated thickness of White Porphyry 
is now found between the Blue Limestone and the overlying Weber Grits. 
The fused rock-mass at the time of its eruption must have been nearly fluid 
enough to obey the laws of hydrostatic pressure, in accordance with which 
the force applied to it at any point would be equally distributed throughout 
its mass and equally transmitted in every direction against its boundary 
walls. As long, therefore, as it retained the fluid condition, this force would 
be expended, not only in raising the beds immediately over the vent, but 
in spreading open the strata at as great a distance from this vent as the fluid 
mass could penetrate. The fluid condition was not, however, retained indefi- 
nitely, but the mass cooled gradually, and, in cooling, became solid and no 
longer capable of transmitting the hydrostatic pressure ; therefore, the force 
available for prying open the strata became gradually less as the distance 
from the vent increased, and the distance by which the strata were forced 
apart at the vent may be assumed as the measurement of the maximum force 
exerted. It has been seen that the thickness of beds above this horizon to 
the top of the Cretaceous, which is the series assumed to have accumulated 
before the igneous rocks were injected or intruded, is estimated at 10,000 
feet. It is possible that at the time of intrusion these beds were still under 
water, in which case the weight of the water should also be added. But 


this is too uncertain a matter to enter into even so crude a calculation as 
the present, and may therefore be neglected. The average specific gravity 
of these beds may be assumed at 2.50, as the upper beds were probably 
somewhat lighter than the average of those observed in this region. A 
cubic foot would therefore weigh 155.8875 pounds, and 10,000 cubic feet 
1,558,875 pounds, which is the theoretical pressure exerted by gravity on 
each square foot of surface, and to raise this 1,500 feet would require a 
force of 2,338,312,500 foot-pounds exerted on each square foot of surface. 

The above figures are to be considered rather as an indication of the 
magnitude of the subterranean forces involved than an actual value of any 
particular force, since the assumptions on which they are founded cannot be 
mathematically proved. For instance, on the contraction theory of the 
folding of the beds, the tangential strain to which they were already sub- 
jected may have been sufficient to produce a tendency in the beds them- 
selves to split apart, and thus in part have counteracted the theoretical 
pressure exerted by gravity. 

Mathematical demonstrations, as applied to geological phenomena, are 
at best of very doubtful value, owing to the impossibility of obtaining data 
or measurements of an exactness that may be considered of mathematical 
accuracy, and it often occurs that such demonstrations, which undoubtedly 
display a high order of mathematical ability on the part of their author, are 
comparatively worthless, or even misleading, owing to his assumption of a 
premise which cannot be proved to be true. 

Source of intrusive force. What may have been the impelling force which 
brought the fused material to its present position is evidently a purely specu- 
lative question, and therefore hardly appropriate to be discussed here. What- 
ever it may have been, it was undoubtedly of the same nature as that which 
has caused flows upon the surface. Much ingenuity has been displayed by 
theoretical geologists in discussing the source of volcanic energy, but in 
the present stage of experimental or synthetic geology it is impossible to 
find direct proofs for or against their views. The theory advanced by 
Clarence King (op. cit.) is among the latest, and is deserving of considera- 
tion because of his long and varied field experience. It may be stated in a 
crude, brief way as follows : Starting with the assumption of a solid interior, 


he shows that the increment of heat and the increment of pressure from 
the surface toward the interior of the earth are not the same, but may be 
expressed by two curves which would cross each other at a given depth. 
Under normal conditions, by the time the temperature in depth has increased 
to the ordinary fusion point of rock masses, the pressure has also increased 
to such a degree as to raise the fusion point of these rock masses, so that it 
is no longer possible for them to fuse. This he considers the permanent 
condition of the earth below the point of junction of the curves of temper- 
ature and pressure. Now, if for any reason the pressure is suddenly 
decreased, as it would be by the removal of a considerable weight of rock 
from the surface over a given area, and if this removal is more rapid than 
the change of temperature, which owing to the low conductivity of rocks 
must be very slow, fusion would set in and a subterranean lake of molten 
rock be formed. He conceives that for mountain areas the removal of 
large amounts of rock material by erosion would be relatively rapid enough 
for this purpose. Upon the thus melted mass there would be exerted the 
pressure of the rocks above it, and probably also an additional pressure due 
to expansion of its own mass by fusion, which would force the liquid magma 
toward the surface. 

Why intrusive and not surface flows? The next question that Suggests itself 

is-, why did the fused masses which formed the older rocks stop in their up- 
ward course at a given horizon and spread out there, instead of continuing 
on upwards to the surface, as did the more recent flows I Was it owing to 
a difference in the chemical composition of the magmas from which either 
series were formed or to a difference in the quality and amount of the im- 
pelling force, or, again, to a difference in the resistance offered by the rock 
masses through which they passed ? The first of the three alternatives 
may, it would seem, be at once answered in the negative, since the same 
range in ultimate chemical composition is found in intrusive rocks as in 
recent lavas. The distinction that petrographers have claijned to find be- 
tween the older or intrusive rocks, as a class, and the recent lavas, depends 
on internal structure and the arrangement of the mineral constituents, while 
they acknowledge that the chemical composition of the two classes may be 
practically identical. In this region White Porphyry and Nevadite among 


the acid types of the two classes, and hornblende-porphyrite and hyper- 
sthene-andesite among the more basic, are almost identical in chemical com- 
position. 1 The loci of eruption in either case are not more than ten miles 
apart, and yet in one instance the molten material congealed at a depth of 
over ten thousand feet and in the other at the very surface ; and the result- 
ing rocks are distinct varieties, differing more in the case of the basic ones, 
where composition is more closely alike, than in the acid. 

In a discussion of the origin of a certain group of laccolites, an argu- 
ment has been made in favor of the theory that their laccolitic or intrusive 
character is dependent on the density of the eruptive magma (which is neces- 
sarily a function of its chemical composition) ; that the molten mass would 
stop in its upward progress through the sedimentary strata, when it had 
reached a point at which the average density of the rocks below it was 
greater than, and that of the rocks above it less than, its own average 
density. This argument is, however, materially weakened by the instability 
of some of the premises. First, it is assumed that the density of laccolitic or 
intrusive rocks is less than that of erupted lavas. Even should this prove 
to be true of that group, it would not be a sufficiently wide basis on which 
to found a generalization for laccolitic or intrusive bodies as a whole. Sec- 
ondly, the data used in support of a necessary condition of this argument, 
namely, that "the acidic rock of the laccolites must have been heavier in 
its molten condition than the more basic rock of the neighboring volcanos," 
are, as the author acknowledges, insufficient, even if trustworthy. Aside 
from the value of this argument as such, however, the facts observed in this 
region, as mentioned above, afford a direct proof of observation against it. 
Moreover, as no chemical analyses of the rocks of this jrroup of laccolites 
were made, it is by no means impossible that they are, as a class, much 
less acid than the author supposed. 

Whether or not there was a difference in the impelling force in the case 
of intrusive sheets and of surface flows is a purely speculative ques- 
tion, for which no direct evidence can be obtained. It might perhaps be 
argued that, if the magma from which each of these series was formed 
originated at essentially the same position within the eaith's crust, it would 

'See analyses 1 and 9 and 5 ami 10, Table I, Appendix H. 


have required about the same amount of force to bring the earlier intrusions 
to their place of consolidation, 10,000 feet below the surface, that it did to 
bring the later flows to the surface after the 10,000 feet of superincumbent 
strata had been removed by erosion. 

In regard to the third alternative, it seems that a part at least of the 
reason for the stoppage of the intrusive magma at its present position may 
be found in the resistance to its passage offered by the sedimentary strata. 
If it were a question only of the porphyry sheets above the Blue Limestone, 
it might be assumed that the Weber Grits offered some special conditions 
of impenetrability; but, in point of fact, although intrusive sheets are almost 
always found at this horizon in the Mosquito region, they also occur at 
many other horizons, both above and below. While it cannot be main- 
tained, therefore, that any particular bed offered special resistance to the 
passage of the fused mass, it is not only evident a priori, but supported by 
observations of many of the transverse sheets and dike-like bodies, that a 
continuous and unbroken horizontal rock stratum would offer more resist- 
ance than one that was inclined, broken, or fissured. The molten rock-mass 
would naturally seek joints or fault planes, or, in default of these, follow 
the lines of least resistance along bedding planes. That the intrusive bodies 
are not found following the planes of the great faults of this region would 
in itself be a sufficient proof, were none other available, that these faults 
are subsequent to the intrusion of the older igneous rocks. Taken as a 
whole, it seems evident that the upward passage of a molten stream would 
be much more impeded in a series of horizontal and comparatively unbro- 
ken strata, such as are supposed to have existed here at the time of the 
intrusion of the older rocks, than it would be after they were uptilted, flexed, 
and dislocated, as they were at the time of the eruption of the younger 
series. In the case of each of the three larger masses of true eruptive rocks 
of the region, viz, those of Chalk Mountain, of Black Hill, and of Buffalo 
Peaks, wherever the sedimentary strata are visible in close connection with 
the eruptive mass they are seen to be standing at a very steep angle and 
the eruptive mass has apparently flowed over their basset edges. 

internal structure From this point of view the division of the igneous 
rocks of the region is also well marked, although it is in the nature of 


things more difficult to draw a sharp and definite line of separation than 
in the case of the two characteristics already discussed. It is also to be 
remarked that, whereas these structural distinctions have hitherto been 
considered to be essentially a function of the age of the rocks, the studies 
conducted during the present investigation tend rather to the conclusion that 
these distinctions are primarily dependent on the manner of occurrence of the 
bodies, or, in other words, the conditions under which they consolidated, 
and only secondarily on their age; hence that the age of a rock can only be 
relatively and not absolutely determined by its internal structure and petro- 
graphical constitution. The details of the microscopical structure of the 
various rock species are so fully described and discussed by Mr. Cross in 
Appendix A that only a few of the more prominent characteristics of the 
two types, such as will serve to correlate them with those of other regions, 
need to be given here. The older series are either entirely granular, or, 
where porphyritic, are characterized by the holocrystalline structure of 
the groundmass and an absence of isotropic or amorphous material, when 
examined under the microscope. . Many of the orthoclastic varieties have 
extremely large crystals of that feldspar, which give a striking and easily 
recognizable appearance to the rock masses ; although very prominent 
in Colorado, this peculiarity can hardly be regarded as an essential charac- 
teristic of the type. They are not vesicular or scoriaceous; in other words, 
they present the external characteristics of a rock cooled under pressure. 
The younger type, however, while in exceptional instances almost holo- 
crystalline, generally contains isotropic material or actual glass substance. 
Its orthoclastic feldspars are essentially sanidine, it may be vesicular and 
scoriaceous, and in general carries abundant glass inclusions and bears evi- 
dence, either in its structure or in the constitution of its mineral constituents, 
of having cooled at or near the surface, and consequently more rapidly than 
the older type. In the Mosquito region there is apparently a definite rela- 
tion between age and relatively granular character of the different varieties 
of either type; thus White Porphyry is the most thoroughly granular rock 
among the older series, and Nevadite among the younger. Although the 
relation of pressure and conditions of cooling to internal structure are so 
marked and important in the two great series, or, so to speak, generically, 


the difference of internal structure in a given species, due to difference 
of pressure, is, if it exists at all, so slight as to escape observation. Thus, 
between the lowest body of White Porphyry, which occurs in the Archean, 
and the highest, which is near the top of the Weber Grits (a vertical range 
of about three thousand feet), no essential difference in internal structure was 
detected. It would appear, therefore, that, while very wide differences in 
the conditions of cooling may produce a generic difference between two 
series of rock varieties, the internal structure of a given variety is not 
dependent on those conditions alone, but that the species possesses certain 
essential characteristics of its own which are dependent on other factors. 

While the petrographical studies made in the course of this investiga- 
tion, forming only an accessory and not an essential part of it and being 
confined to a limited area, are not sufficiently complete to form the basis of 
an essential change in the classifications hitherto adopted, they point decid- 
edly to the fast approaching necessity of some essential modification in 
them. Thus, the White and Lincoln Porphyries would a few years ago 
have been unhesitatingly classed by petrographers, from a study of their 
specimens and aside from any field observations on their geological relations, 
as granite-porphyry or mica-granite, and probably of Paleozoic or early 
Mesozoic age, from their resemblance to well-known rocks of that age in 
other parts of the world. The hornblende-porphyrites, on the other hand, 
might from the same standpoint have been classed as Tertiary andesites. 

Orthociastic and piagiociastic rocks. The now universally adopted chemico- 
rnineralogical classification (based on Tschermak's classical studies) of ortho- 
clastic and piagiociastic rocks is one which presents ever-increasing difficul- 
ties of application with the progress of microscopical and chemical investi- 
gation. In the present" instance the older rock series contain relative pro- 
portions of orthoclase and plagioclase feldspar, often so evenly balanced 
that the slight variations in their proportions, which may be found in differ- 
ent parts of what is apparently the same mass, would be sufficient to justify 
the placing of the same rock now in the orthoclastic division now in the piagio- 
ciastic. Again, in those porphyries in which the orthoclastic feldspars have 
developed in large individuals, it is evident that so much orthoclastic mate- 
rial has thus been abstracted from the groundrnassthat, were the latter taken 


as the type of the rock, it would be classed as plagioclastic, while in the 
rock as a whole, or in those varieties in which the large orthoclases have 
not been developed, orthoclase predominates. It is apparent, moreover, 
that, owing to the increased facilities which the microscope now affords for 
the detection of plagioclase among the microscopical constituents of a rock, 
an ever-increasing number of rocks hitherto supposed to be orthoclastic 
will be found to have a predominance of plagioclase feldspars, and that, if 
this distinction remains without modification as a basis of classification, the 
extent of rock species of the orthoclastic type will become more and more 
restricted and eventually rather rare. 1 

Distribution of intrusive rocks in the Rocky Mountains. The older and intrusive 

series of rocks, represented in this region by the porphyries, porphyrites, 
and diorites, form undoubtedly a very large proportion of the igneous 
rocks of Colorado and adjoining regions which have hitherto been classed 
as Tertiary eruptives or as eruptive granites. To how great an extent they 
should be substituted for the latter on the existing geological maps it is not 
yet possible to determine with accuracy, owing to the incompleteness or 
absence of characteristic specimens. An opportunity was, however, offered 
in the case of the Henry Mountains, so ably described by Mr. Gilbert, who 
kindly loaned a considerable number of the actual rock specimens and 
sections, upon which the determinations for his work were founded. These 
were submitted to Mr. Cross for microscopical examination, several new thin 
sections being made by him for this purpose. The results of his investiga- 
tion, although (owing to the incompleteness of the series and the altered 
condition of many of the specimens) not adequate to afford a complete 
characterization of all the rock masses found there, show conclusively that 
they belong to the same structural type as the older intrusive rocks of this 
region. Out of 19 varieties represented by specimens or thin sections, 14 
were found to correspond very closely in composition and structure to the 

'A remarkable instance of this tendency is found in the recent review of rock determinations of the 
fortieth parallel by Messrs. Hague and Iddings (American Journal of Science, xxvii, 453, 1884), which 
shows that in the vast area covered by that survey only a single true trachyte, and that not of the 
most characteristic type, was observed, although iu the original determinations, made iu the light of 
the best petrographical science as it existed twelve years ago, these rooks were supposed to form a 
large and important class there. 



hornblende-porphyrites of the Mosquito Range and 3 differed from the 
Mosquito rocks in containing a peculiar development of augite in the place 
of hornblende. 1 

Mr. Gilbert enumerates various isolated groups of mountains in the 
plateau region the Sierra La Sal, Sierra Abajo, Sierra El Late, and Sierra 
Carriso which, from the description of geologists who have visited them, 
he infers to be true laccolites. He also infers that their rocks are analogous 
to those of the Henry Mountains, which is very likely to prove true in so 
far that what he describes as porphyritic trachyte may correspond to the 
porphyries with large crystals above described. His further generalization 
that the two types of mountain structure, the laccolitic and the volcanic, 
necessarily involve two chemical types of rock, the one acidic, the other 
basic, is, as shown above, not authorized by the observed facts. It might 
fairly be reasoned that the more acidic lavas, when intrusive, owing to their 
greater viscosity, would tend to form thick, dome -shaped masses like his 
laccolites, rather than basic lavas; but even this tendency is not without its 

It is the intrusive quality, not the relative acidity or basicity of the 
magma, to which the characteristic structure of this rock type is due. 

Dr. Peale 2 has further extended the probable development of intrusive 
bodies, more or less analogous to the laccolites in form, but furnishes no 
decisive determination of their petrographical structure or composition 
From specimens seen or actually collected by the writer, it may be stated, 
however, as a fact about which there can be little question, that the type 
of intrusive rock represented by the older series is extensively developed 
between the North and Middle Parks, in the Middle Park, and between 
the Middle and South Parks, that it forms the mass of Spanish Peaks, and 
occurs in enormous developments in the Gunnison region, where the vari- 
eties characterized by large feldspars cut across Cretaceous strata. Similar 
bodies also exist beneath the more recent lavas of the San Juan region, 
which lends probability to the supposed similarity of the rocks forming the 
isolated mountains of the Sierra El Late, Sierra Carriso, and others. 

'Mr. Cross's detailed description of tbese rocks will be found at theeud of Appendix A. 
*" On a peculiar type of eruptive rocks in Colorado." Bulletins United States Geological and Geo- 
graphical Survey, Vol. ill, pp. 551-5C4. 


Contact metamorphism. There is a notable absence of caustic phenomena 
in this region, either on the inclosing or the included sedimentary rocks at 
their contact with the intrusive masses, such as are generally supposed to 
accompany the eruption of igneous rocks. 

In the case of such numerous and large bodies they might naturally 
be expected to be exceptionally frequent and well marked, since the eruptive 
masses must have retained great heat for an unusually long time on account 
of the depth at which they were consolidated. Perhaps the absence of any 
evidence of fusion in these rocks might be explained on this very ground, 
that at that depth the pressure was so great that the fusion point was con- 
siderably raised, and hence a temperature sufficient to hold in a molten 
condition the mixed material already fused would be insufficient to melt 
homogeneous and by themselves comparatively refractory rocks, like sand- 
stones and dolomites. However this may be, nowhere was any evidence 
of fusion observed in the sedimentary rocks, even in the case of very small 
fragments entirely included in the eruptive rock. Even in the dikes of por- 
phyrite cutting through the Archean, in which inclosed fragments of coun- 
try rock, generally of small size, are particularly abundant, neither quartz, 
granite, nor gneiss, of which these fragments generally consist, shows any 
alteration at the contact, though the porphyrite material often tills small 
cracks in them, showing that it was in a thoroughly fluid condition at the 
time they were caught up. Such alteration as was found could more 
readily be ascribed to the combined action of heat and water than to heat- 

On the other hand, the reflex action of the colder sedimentary rocks on 
the eruptive mass is generally noticeable, and is such as is ordinarily found, 
showing itself in a fine-grained or even compact structure for a few inches 
or more from the contact, and in a somewhat different arrangement of the 
mineral constituents of the rock. It is much more prominent in the narrow- 
dikes than in the large intrusive sheets, but occurs at both upper and lower 
contacts of the latter, and may also be detected around the included frag- 
ments. Even in these cases, however, there was no appearance of vitrifi- 


Instances of regional metamorphism are not wanting. Sandstones are 
changed to quartzites, dolomites frequently into marbles and less often more 
or less serpentinized; but these changes cannot be assigned to the direct 
action of heat, since they are in no sense contact phenomena. Their devel- 
opment is local and irregular, extending over considerable areas, where 
there is no actual contact of the altered beds with intrusive rocks, and, on 
the other hand, being more generally absent from the actual contact with 
these rocks. 

Non-absorption of sedimentary rocks by erup'.ive masses. Another important obser- 

vation in regard to these intrusive bodies, and in one sense a corollary of the 
above statements, is the fact that, although they have split apart and pried 
open the sedimentary strata and caught up or entirely surrounded both 
large and small fragments of sedimentary rocks, there is no evidence of 
their having absorbed or assimilated within themselves by actual fusion 
any portion of these sedimentary rocks; certainly not any considerable 
masses thereof. Not only are there no relics of fusion at the present con- 
tact, as there necessarily would have been if a portion had already been 
fused, but in reconstructing the sections on actually measured profiles there 
is no portion of the sedimentary strata missing, which cannot be accounted 
for by erosion. Along the contact surface the fused mass has cracked off 
fragments, often quite small, which have consolidated again into a sort of 
breccia ; again, the thinner sheets have sometimes Itent back and contorted 
a stratum of limestone or quartzite at the end of the flow or as it crossed 
from one bed to another; but of fusion, as already stated, there is no sign. 
I have insisted on this point because the question of the capability of 
an igneous mass to absorb, or eat up as it were, the sedimentary or even 
already consolidated igneous rock through which it passes, is one which 
has always interested me, and for which, in a field experience of over fifteen 
years, largely among eruptive rocks, I have vainly sought for demonstrable 
proof. It is customary among geologists to draw their ideal underground 
sections of igneous masses as if this capability were unlimited, and geo- 
logical text-books seem to tacitly assume that it is so, without offering an 
explanation of how it is possible or the grounds on which the assumption 
is made. 


So far as I know, the English geologists are the only ones who have 
met the question distinctly and have brought forward instances in nature to 
prove that igneous eruptions have eaten up practically unlimited amounts 
of sedimentary rocks. These instances are the so-called granite bosses in 
Ireland (Mourne Mountains), Scotland, and England (Devon and Cornwall), 
cutting through upturned Cambrian and Silurian rocks, which are compar- 
atively undisturbed by the eruption and maintain their normal strike up to 
these granite masses on either side. 1 

Professor Geikie goes so far (op. cit , p. 550) as to ascribe the vari- 
ability in composition and structure of intrusive masses to involved and 
melted-down portions of sedimentary rocks. It would be presumptuous to 
doubt the correctness of the field observations on which these generaliza- 
tions are founded, and yet it is not only possible, but has sometimes come 
under my observation that a granite boss has been found protruding through 
a given rock or series of rocks, and therefore been judged by the geologist 
who examined it to be younger than the latter, whereas, in fact, the reverse 
was the case, and the latter rock had been deposited, or had flowed, around 
an already-existing granite protrusion. In many cases it is difficult to obtain 
direct proof whether the protruding or the inclosing rock is the older, and 
in such a case the probability one way or the other may be dependent on 
this very question of the capability of igneous rocks to assimilate large 
masses of sedimentary rocks. 

A case in point is the granite body of Little Cottonwood canon, in the 
Wasatch Mountains, of which a section some seven miles long has been 
exposed by the erosion of the canon. The present outcrops of the body 
occupy an area whose dimensions may be roughly stated as 7 by 15 miles ; 
and a thickness of some 5 miles of sedimentary rocks abuts against its 
northern side, the upper members sweeping round and in part covering its 
eastern portion, and continuing southward in an almost horizontal position. 
There is no special disturbance of these beds in contact with the granite ; 
so far as observed, they follow the normal dip and strike induced by the 
dynamic movement of the region. Neither are there any masses or fragments 
of sedimentary rocks included in the granite. Regional metamorphism exist* 

'A. Geikie: Text Book of Geology, pp. 541 et seq. London, 188:3. 


in the changing of sandstone to quartzite and of limestones to marble, but 
these are by no means contact phenomena, and occur as often, if not oft- 
ener, at considerable distances from the granite as in direct contact with it. 
Porphyry dikes also cross the sedimentary strata in the vicinity, but these 
have no more necessary connection with the granite than have the neigh- 
boring bodies of volcanic rocks. They are not direct offshoots from it, and, 
so far as their manner of occurrence and structure go, may bear the same 
relation to it that the porphyries of the Mosquito Range do to the Archean 
eruptive granite. When I examined this region on the Exploration of the 
Fortieth Parallel, my first impulse, guided by my teachings as a student of 
geology, was to consider the granite an intrusive mass cutting Carboniferous 
strata ; it was, however, difficult to conceive that it should have eaten up 
over five hundred cubic miles of sedimentary rocks without leaving some 
more definite evidence of this action than it has. This, together with other 
considerations, led me, after a careful weighing of the evidence, to the view 
that the granite must have been erupted in Archean time, and that in the 
ocean of the Cambrian and subsequent periods it formed a submerged reef 
around which the sedimentary beds were deposited. Professor A. Geikie, the 
English geologist, whose eminent ability none can recognize more full}" and 
heartily than I do, after a visit to the region, occupying only a few days, 
decided promptly that my view was wrong, and, evidently basing his opinion 
on the granite bosses of his own country, has published it in his text-book 1 
as an instance of Post-Carboniferous granite. While, owing to the necessa- 
rily hasty character of reconnaissance work like that of the fortieth parallel, 
it is very possible that a more detailed study might lead us to modify our own 
views, especially in regard to so complicated a district as that in question, 
I should still be unwilling to admit, even at the instance of so experienced 
a geologist as Professor Geikie, that the Cottonwood granite can be Post 
Carboniferous, even if my only reason were that I do not admit the possi- 
bility that the granite had eaten up or assimilated this enormous mass of 
sedimentary rocks without leaving any trace of fusion on the adjoining 
rocks, any incompletely assimilated portions within its own mass, or with- 
out showing in its own structure and composition any marked variation from 
that of the normal rock. 

'Op. cit., p. 646. 


It seeins to me that there is a marked distinction between the meta- 
tnorphism that is found in regions where igneous rocks abound (and which 
is generally admitted to be the result of the combined action of heat, press- 
ure, and water) and that which involves the entire absorption and assimila- 
tion of foreign rock masses into the substance of the igneous mass itself. 
The former in its extreme phase supposes a simple rearrangement of the 
materials of a rock, a change in their form without any essential change in 
their chemical composition, and involves at most the bringing of them to a 
viscous state, not to that of fusion. The latter must be a dry process and 
involves a fusion of the foreign materials as complete as thai of the original 
magma in the deep-seated source from which it came. For fusing the 500 
cubic miles of sedimentary beds supposed to have been assimilated by the 
Cottonwood granite body an enormous amount of heat must have been 
abstracted from that body. Now, to have this amount of heat to yield up, 
and yet to be able to maintain itself in a state of fusion long enough to 
crystallize in the same way that it would without this addition of foreign 
material, supposes an amount of original heat stored up within its mass 
that ought to have vitrified some of the rocks through which it passed. 
It is not difficult to conceive of such heat in the deep-seated source from 
which the igneous rocks came, but that it should still exist in these rocks 
when they have reached the point where they are ready to solidify, and 
which may be assumed to be near the limit that this heat would carry them, 
seems highly improbable. The only cases of actual vitrification of inclosed 
fragments in igneous rocks that I have read of have been in recent volcanic 
rocks, where the fragments were extremely small. 

As suggested above, the pressure under which the intrusive rocks of 
the Mosquito Range were consolidated would necessitate a higher tempera- 
ture to produce fusion. In the case of the Cottonwood granite the pressure 
under which consolidation took place and the consequent temperature of 
the fusion point must have been greater still. But the Mosquito porphyries 
retained a very fluid condition, and therefore a temperature higher, as com- 
pared with the fusion point, than the Cottonwood granite, for a very long 
time, since they were spread out in thin sheets and ramifying bodies in 
every direction at considerable distance from the central mass, while the 


Cottonwood granite, as far as can be seen, formed only a single massive 
body without ramifications. The porphyries must therefore have had more 
superfluous heat than the granite to devote to the work of melting np the 
included masses of sedimentary rocks, and one can see here, as one cannot 
in the granite, that such masses were actually caught up and included in 
the fused rock. It would be fair to assume, therefore, that in this case rela- 
tively larger amounts of sedimentary rocks would have been fused and that 
the evidence of such fusion would be more apparent. 

In the present condition of microscopical investigation we may trace the 
development of one mineral from another and detect its most minute altera- 
tion, either by fusion or by chemical interchange; and, had any of these sedi- 
mentary rocks been assimilated into the igneous mass, it would seem hardly 
possible that every trace of the process should have escaped our observa- 
tion in the thousands of rock sections that have been examined. In point 
of fact, however, although in the case of the porphyrite dikes the eruptive 
material is found to fill minute cracks in the inclosed fragments of Archean 
rocks, there could be detected no evidence of fusion on either adjoining or 
inclosed sedimentary rocks. In the eruptive rocks themselves, moreover, the 
alterations of mineral constituents are all the result of secondary processes 
after the mass had fully cooled and crystallized. 

The testimony of the chemical composition of these rocks is, so far as 
it goes, equally opposed to the supposition that foreign matter has been 
assimilated by any of these intrusive bodies. Of White Porphyry too few 
specimens were analyzed to afford a decisive test; but it is to be remarked 
that the two specimens (see Table II, Appendix B) which show an abnor- 
mally high percentage of silica are from the London and New York mines 
and are extremely decomposed and altered, a secondary action which has 
decreased the proportion of more soluble basic constituents and correspond- 
ingly increased the percentage of silica. Of the Lincoln or Gray Porphyry 
six specimens from different bodies show an average of 68.08 per cent, of 
silica, with an extreme variation from this average of 2.63. For the com- 
bined alkalies, three specimens show an average of 6.14 and an extreme 
variation of 0.86; and for lime and magnesia combined, three specimens 
show an average of 4.03, with an extreme variation of 0.19. 



The Cottonwood granite, which, on the supposition advanced by Pro- 
fessor Geikie, must have taken up an enormous amount of silica, lime, and 
magnesia, shows, however, no abnormal amount of these constituents in its 
composition, which is that of a normal granite rather rich in plagioclase. * 

1 The composition of this granite, as given in Exploration of the Fortieth Parallel, Vol. II, p. 
357, is as follows : 

SiOa 71.78 

A1 3 O 3 14.75 

FeO 1.94 

MnO . . 0. 09 

CaO ............ 

MgO ........... 

K 2 O 

. 0.71 
. 3.12 

. 4.89 
. 0. 52 

100. 16 








Introduction 319 

Classification of Mosquito Range eruptives 322 

Older eruptives 323 


Mount Zioii Porphyry 323 

White or Leadville Porphyry . - 324 

Pyritiferous Porphyry 326 

Mosquito Porphyry 327 

Lincoln Porphyry 328 

Gray Porphyry 330 

Diorite ' 333 

Quartz-mica-diorite 333 

Hornblende-diorite 333 

Augite-hearing diorite 334 

Porphyrite 334 

Principal group 335 

Sacramento Porphyrite _,. 341 

Silverheels Porphyrite 342 

Miscellaneous porphyrites 343 

Younger ernptives - 345 

Rhyolite 345 

Chalk Mountain Nevadite 345 

Black Hill rhyolite 349 

McNulty gulch rhyolite 350 

Empire gulch rhyolite 351 

Other rhyolites 352 

Rhyolitic tufa in South Park 352 

Dike in Ten-Mile amphitheater 352 

Breccia in the Eureka shaft 352 

Quartziferous trachyte 352 

Andes! te 353 

Pyroxene-bearing hornblende-andesite 353 

Hyperstheue-andesite 354 

Tufaceous audesites 354 

R&junie" 354 

The rock structures observed 355 

Individual rock types 355 

Mutual relations of rock types 356 

Rock constituents 350 



Re'sunKS Continued. 

Decomposition of rock constituents 356 

Negative observations 357 

Chemical composition 357 

Notes on the Henry Mountain rocks 359 

Hornblendic rocks 359 

Augitic rocks 361 

Resume 1 362 




The eruptive rocks of the district embraced by this report are naturally divisible 
into two groups, according to age. Although the age of neither group can be exactly 
defined in geological time, the larger and more important one is unquestionably 
older than the period of disturbance which produced the great faults and folds described 
in other parts of this volume, while the other group is younger. In this district, rocks 
of the former group penetrate the Tipper Coal Measure strata; in adjoining regions 
they occur in similar manner in the Trias; and masses of nearly identical character 
are found in the Cretaceous of districts not far removed from the Mosquito Eange. 
The conclusion that the rocks of the older group are of late Mesozoic age seems war- 
ranted by all that is known concerning their occurrence. In regard to the period of 
dynamic disturbance, it has already been stated in Chapter II that the known evidence 
places it at the beginning of Tertiary time. 

It is plain, then, that the rock groups mentioned might be considered the direct 
equivalents of the Tertiary and Pre-Tertiary divisions of many writers, but it is thought 
best to refer to them simply as the older and the younger groups, and by this division 
it is intended to express merely the actual relationship as to age which is shown 
by the observed occurrences. The question concerning the possible influence of age 
upon the structure of these groups cannot be fully discussed at the present time, 
because the rocks of other districts in Colorado, the study of which has been under- 
taken, form with those of the Mosquito Range a connected series, requiring a correla- 
tion of observations upon all of them before justifiable conclusions can be drawn. 

All of the older eruptives and some of the younger series are fully crystalline, 
although few of them are typical granular rocks, and the structural forms presented 
are such as render advisable some statement as to the sense in which the terms " gran- 
ular" and " porphyritic" are used in the descriptions that follow. When these terms are 
applied with the old and natural meaning, to designate certain universally recognized 
rock structures, it is probable that the groups formed by the application will be prac- 
tically the same, whether the attempt is made to accurately define the boundary line or 



not. All consider granite as a typical granular rock; and tbat rock which would be 
cited by any one as typical of the porphyritic structure could scarcely be placed else- 
where under any existing definition. This latter assertion is at least true now that 
Roseubusch has withdrawn his earlier definition, 1 by which the presence of some 
amorphous matter in the groundmass was made essential to a porphyry. The new 
ground taken by Kosenbusch 2 in regard to the essential difference between the gran- 
ular and porphyritic modifications of eruptive rocks seems to the writer open to some 
serious objections, although the great value of many of the points so clearly presented 
must be gratefully acknowledged by all. While a full discussion of the question can- 
not be entered upon in this place, the chief objections to the new definition may be 
briefly stated. 

If the writer correctly understands the position taken by Kosenbusch in his essay 
upon the essence of the granular and porphyritic rock structures, the latter wishes so to 
redefine the terms "granular" and "porphyritic" that they shall henceforth indicate 
genetic and not structural relations. It is claimed that the typical structures hitherto 
designated by these terms have their origin iu the history of each individual rock 
mass; the granular rocks having come to complete solidification iu the course of what 
may be termed a single phase; the porphyritic types, on the other hand, having passed 
through two phases, in the second of which th groundmass was formed the matrix 
for the crystals of the earlier phase. The genetic groups thus outlined are to replace 
the structural ones, while the terminology is to remain the same. 

The first objection to be raised is that a new division of eruptive rocks accord- 
ing to a genetic principle does not in any way destroy the purely structural groups 
already existing, even if the divisions produced by the two principles are exactly 
coincident in extent. It will still be desirable and necessary to refer to rock struct- 
ures independently of genetic connections, and the terminology of the science is not 
simplified but rather complicated by the application of a given term in two distinct 
senses. Granular cannot be logically used with a genetic meaning while, at the same 
time, it is desirable to apply it in accordance with existing usage as a purely struct- 
ural term. In the second place, it seems a matter for debate as to whether the groups 
formed on the new principle are coincident with the structural ones. If not, we surely 
cannot cover them by a single definition, nor use the same terms in their description. 

That the new definitions, when logically applied, do produce divisions widely 
different from the corresponding structural groups is well illustrated in the case 
brought up by Eosenbusch himself, in a passage of which the following is a free trans- 
lation : 

If we follow iu thought the process of granite formation, we reach at length a point, after the 
separation of ore-grains, apatite, zircon, biotite, hornblende, or angite, and a part of the feldspars, 
where, between the ready-formed mineral particles which are to make up the mass of the rock, a very 
fluid, acid residue remaius, out of which some feldspar and quartz are yet to be formed. If now, through 
any cause, the solidification of the rock be suddenly interrupted at this point, the residue will solidify 
as amorphous substance (it might under certain conditions be spherulitic or even granophyritic) and 
we have thus a granular mixture of the granite minerals (with the exception of quartz) and irregular 

1 Physiographic der massigen Gesteine, pp. 8fi, 87. 

'"Ueberdas Wesen der kornigen and porphyrischeu Structur bei Masseugesteinen." Neues 
Jahrbuch ftir Mineralogie, etc., II, 1, 1882. ' 


patches or particles of a very acid glass a case described by G. voru Rath in a so-called trachyte from 
Monte Amiata, in Tuscany. Such a rock can only be designated as a granular rock which is not entirely 
holocrystalline. On the other hand, if the rock contains quartz among its crystalline particles, then it 
may no longer be regarded as granular, but rather as a porphyritic rock. 1 

According, therefore, to the new rule, strictly applied, we may have a granular 
rock containing glass. In the case cited the glass is described as in isolated particles; 
but the classification could not have been different had it appeared as a base holding 
and cementing together the mineral grains, neither can the amount of this glass be 
restricted under the considerations which gave rise to the definition. A rock of the 
orthoclastic series, containing crystals of ore, biotite, apatite, plagioclase, and some 
orthoclase, imbedded in glass or microfelsite, which might compose more than half of 
the mass, would still be a granular rock, while, had the crystallization proceeded further 
and some quartz been added to the other minerals, the product would have been a 
porphyry. Again, iu referring to the observed difference between diabase and gabbro 
resulting from the formal development of the feldspars, Eoseubusch remarks that this 
difference is only an apparent one, if the essence of the diabase structure be considered 
as lying iu irrelative age of the feldspars and not in their form. 2 Yet this formal dif- 
ference still exists and must be described; but, if Roseubusch's definitions be adopted, 
it cannot be described as structure. 

These instances have been considered somewhat in detail, to show clearly the cor- 
rectness of the statement that Roseubusch desires to replace the structural groups by 
purely genetic ones, and also to show that the two divisions are not coincident in 
extent. In regard to the latter point it seems to the writer that it may fairly be ques- 
tioned whether all granular rocks are the result of one phase and whether ail porphy- 
ritic rocks have required two phases of consolidation. 

Finally, the great precision aimed at by Professor Rosenbusch iu his new defi- 
nitions seems to be unnatural. Rock groups bleiid insensibly in all directions; there- 
fore sharp boundary lines are arbitrary and undesirable. 

In the following rock descriptions the terms "granular" and "porphyritic" are 
used in the purely structural sense. Were the genetic principle applied the grouping 
would be the same. 

1 " Verfolgen wir in Gedanken den Act der Grauitbildung in seiiiem Verlaufe, so wird uach 
Ausscheiduug der Erze, Apatite, Zirkone, Biotite, resp. Amphibole oder Pyroxene, und eiues Theils 
der Feldspathe ein Stadium eintreten, wo zwischen den ausgeschiedenen, die fertige Hauptuiasse des 
Gesteins bildendeu Gemengtheilen in unregelmiissigen Partien eingekleuimt ein sehr acides Magma 
vorhanden ist, aus welchem sich der letzte Rest der Feldspathe tind der Quarz auszuscheideu batten. 
Denken wir uns nun durch irgend welche Ursache an dieser Stelle den Bildungsprocess des Gesteins 
plotzlich uuterbrochen, so wird der Rest von Mutterlauge amorph erstarren (er kouute unter Umstandeu 
auch spbarolithisch, ja granophyrisch erstarren) und wir erhalten so ein korniges Gemenge der Granit- 
miiicralien (mit Ausnahme des Quarzes) und tmregelniassige Brocken uud Partien eiues sehr saurcn 
Glases bekanntlich ein Fall der uach G. vom Rath's Beschreibung bei eiuem sogenaunten Trachyt 
vom Monte Amiata in Toscana vorliegt. Ein solches Gcstein kann nur als eiu korniges Gestein mit 
nicht ganz holokrystalliner Ausbildung bczeichnet werden. Enthielte dagegen das Gestein uuter den 
krystallinen Ausscheidungen auch den Quarz, eo ware es danu iiicht mehr als ein komiges, soudern als 
ein porphyrisches zu betrachten." Op. cit., p. 15. 

2 "Wenn man das Wesen dtr Diabasstructur nicht iu der Form, sondern in dem relativen Alter 
der Feldspathe sieht." Op. cit., p. 8. 


Classification of Mosquito Range eruptives. The eruptive rocks Of the Mosquito Range 

are classified as belonging to the following groups : 

/ Quartz porphyry. 
Older . . ) Dioritc. 

' Porpliyrite. 

Younger { 


For the reasons given iu the chapter on rock formations, the possible eruptive 
granites of the Archean areas are not included in this discussion. It is at once noticed 
that basic eruptives, such as diabase or basalt, do not occur in this region, and even the 
andesites above mentioned are only found outside the area mapped. 

Nearly all the important rocks of the district are described as quartz-porphyry 
or as porphyrite. Of these two classes there are several marked types, and they are 
so connected by intermediate or transition forms as to build an almost complete series, 
uniting the dissimilar extremes. The treatment of these rocks in the present chapter, 
which has been revised in the light of experience gnined in adjoining districts, is some- 
what different from that at first adopted ; hence a few minor discrepancies may be noted 
between the classification here given and that indicated by the coloring of the map 
and the text of the main work. The map was engraved and colored before this infor- 
mation from other districts was obtained, and could not, therefore, bo changed ; the 
general text is, however, consistent with the divisions of the map. The inconsisten- 
cies alluded to are really of but little moment, as they relate to certain more or less 
questionable forms near the line between quartz -porphyry and porphyrite, which, taken 
by themselves, might readily be differently classed by different persons. The changes 
are introduced here for the sake of preserving, as far as possible, a uniform system in 
this and in forthcoming reports on adjoining districts. 



This rock occurs in the masses of Mount Zion and Prospect Mountain and is des- 
ignated by a special coloring upon the detailed Leadville map, while it is united with 
the White Porphyry on the map of the Mosquito Range. 

In structure it resembles a fine-grained granite at first glance, there being but few 
biotite leaves, with occasional feldspar and quartz crystals, which by reaching a diam- 
eter of three or four millimeters become conspicuous in the mass of the rock. When 
the rock is fresh the naked eye easily distinguishes many quite uniformly small quartz 
grains imbedded in the feldspar, which is the chief constituent. Biotite is uniformly 
but sparingly present in small, irregular leaves. 

Microscopical By the aid of the microscope the following constituents are found, 

named in order of their formation: Zircon, 1 magnetite, apatite, biotite, plagioclase, 
orthoclase, and quartz. 

With a low power of the microscope the chief part of the rock is found to consist 
of an irregular granular mixture of orthoclase and quartz, the latter occurring in 
roughly rounded grains 0.3 mm to 0.7 mm in size, which often seem inclosed in the more 
irregular and frequently larger grains of orthoclase. The presence, in almost every 
grain of these two minerals, of plagioclase inicrolites having a prismatic habit with 
apparently somewhat rounded terminations, and averaging O.l mm in length by 0.01 mm 
to 0.03 mm in width, shows their coincident formation. These microlites, which con- 
sist of from two to five laminae, are very numerous and form the most character- 
istic constituent of the rock. Plagioclase grains occur, corresponding in size to those 
of orthoclase and quartz, but they usually show some crystal outlines, and through 
their freedom from the microlites the correspondence of these grains to the larger, 
stout crystals, which are sometimes 4 millimeters in diameter, seems clearly estab- 
lished. Tbe total absence of the microlites, the difference in form, and the larger 
angle of extinction, reaching in some observed cases 20 either side of the twinning 
plane, show plainly that these crystals represent an earlier and doubtless more basic 
variety of plagioclase thau the microlites. The larger crystals are not abundant, and 
are seldom prominent in the hand specimen. Biotite is never developed in crystal 

1 In nearly all the rocks of this district a mineral, presumably zircon, has beeu found. Its identity 
has been proven iu a rock from the Ten-Mile district, chemically and crystallographically. 



form, and is usually much altered. The three accessory minerals are sparingly present, 
apatite especially so. In none of the sections examined is there any finer-grained 
interstitial matter. 

Alteration The decomposing agencies acting upon the Mount Ziou Porphyry seem 
to have been particularly favorable to the formation of Muscovite, which is the end prod- 
uct of the alteration of the biotite, as well as the immediate one of that of orthoclase 
and plagioclase. In the latter two minerals the process takes place in the usual way, 
and in the extreme decomposed state each grain and inicrolite not wholly inclosed in 
quartz is replaced by a brilliantly polarizing aggregate of minute, colorless, but 
lustrous leaves. In the case of the biotite there are visible transition stages. Ore 
particles and yellow needles (rutile ?) are first formed, aud the biotite passes into a 
yellowish-brown, faintly-polarizing, unknown substance, which soon gives way to a 
mica indistinguishable from the product of the adjoining feldspars. Occasionally 
pure leaves of muscovite are found in quite fresh rock, but, as they always increase in 
quantity in more decomposed specimens, their secondary origin is probable. No other 
secondary product of importance remains, in the advanced stages of decomposition. 
Specimens of Mouut Zion Porphyry which are bleached through the disappearance 
of the biotite become indistinguishable from White Porphyry. (See p. 7G.) 


On account of its relation to the ore bodies, its peculiar mode of occurrence, the 
large area in which it is found, aud its petrographical interest, the White Porphyry 
must be regarded as the most important eruptive of the district, and it will be described 
in considerable detail. 

Macroscopical In its most typical form it is a nearly white, compact or finely 
granular rock, which at first glance seems to be homogeneous, but under close exami- 
nation usually discloses a number of small feldspar crystals, and, scattered irreg- 
ularly through the mass, not unfrequeutly, double pyramids of quartz. Hexagonal 
crystals of dark brilliant muscovite may occasionally be seen, but this is probably 
secondary, as are, very certainly, the clusters of pearly leaves of the same mineral, 
which are characteristic of the rock in some places, as in California gulch, on Lamb 
Mountain, an.l in the intermediate region. The total absence of biotite aud bisilicates 
makes the rock seem dull white, except when stained by secondary iuliltration prod- 
ucts, aud decomposition in the ordinary way only makes the rock seem more homo- 
geneous aud compact than before. Upon the contact with the wall-rock or iu some 
of the more narrow dikes the White Porpyhry is found to contain more numerous 
crystals of quartz and feldspar, imbedded in a very compact groundmass [235].' 

Through decomposition the rock assumes iu some places a granular appearance, 
as if composed of small, worn grains, 3 but no corresponding microscopical structure 
can be seen. 

1 The collectiou numbers of particular specimens will be inclosed in brackets. 

2 The structure referred to is illustrated by specimens from the StoauiiiH O'Brien [:tt], Robert Emmet 
[33a], Little Pittsburgh [32a], and Katie [33c] claims. 


Microscopical The essential constituents of the White Porphyry are plagioclase, 
orthoclase, and quartz, developed in a remarkably uniform-grained mass, in which lie 
occasional crystals of one or more of the same minerals. Orthoclase seems to pre- 
dominate, but never very greatly, and the chemical analysis confirms this view. Com- 
pared with the Mount Zion Porphyry, it is found that plagioclase occurs also in microlitic 
forms, but less alnmdantly, and in some of the more compact modifications may be wanting. 
Biotite, which was present in the Mount Ziou rock, has never been seen in any of the nu. 
merotis specimens of White Porphyry collected, nor was it ever noticed in the field, not- 
withstanding the fact that much of the rock seems quite fresh, judging from the condi- 
tion of the feldspars. As the White Porphyry seems in all other respects to be very 
closely allied to the variety mentioned, it is to be particularly noted that many of the 
muscovite leaves are found to contain yellowish needles (rutile?) or stout crystals (ana- 
tase ?) directly comparable to those resulting from the decomposition, of biotite in the 
Mount Zion and other porphyries. It is therefore probable, in spite of the singular 
absence of intermediate alteration products, that a part of the muscovite in the White 
Porphyry came from biotite. Magnetite is found very rarely, and apatite scarcely 
more frequently, while zircon in minute, brilliant crystals is quite abundant. 

The size of the grains is sometimes but little below the power of vision of the 
naked eye, and they might frequently be distinguished were it not for the decom- 
position of the feldspars. In numerous instances, however, usually in dikes or con- 
tact specimens, but sometimes in large masses, the texture becomes so fine that it 
is beyond the power of the microscope to identify separate granules as quartz or feld- 
spar, and the mass thus becomes cryptocrystalline. In all such cases the structure 
remains evenly granular, there being no tendency towards a development of indistinct 
fibrous matter, nor does any portion appear amorphous, or, more correctly, isotropic. 
A few minute, irregular inclusions are usually visible in the larger quartz grains, some 
of them being undoubtedly fluid, while the others are not recognizable. No glass is 
determinate, and the minute, dark interpositions in the feldspar are probably second- 
ary forerunners of the coming decomposition. 

Alteration. Here, as in the Mount Zion rock, the conditions have favored the 
production of muscovite from all changeable constituents. Only in comparatively 
rare cases do calcitc and kaolin appear. Many of the specimens collected are very 
much altered and show when examined in polarized light under the microscope a 
number of irregular quartz grains, imbedded in a brilliant, variegated mass of minute 
muscovite leaves. Little aggregates representing the original rnicrolites of plagioclase 
penetrate the quartz grains in every direction. It is owing to this decomposition that 
the quartz is ordinarily invisible in hand specimens, as the muscovite leaves envelope 
each grain so closely that fracture does not separate them. The leaves of muscovite 
are so very small that the characteristic luster is seldom detected without close exam- 

Chemical composition of Mount Zion and White Porphyries. Analysis I, below, was 
made by L. G. Eakins, upon a fresh, specimen of Mount Zion Porphyry from the 
Little Harry shaft, Prospect Mountain [24a]. Analysis II by W. F. Hillebrand, 
upon a typical specimen of White Porphyry from the quarry in California gulch at 



the southwest base of Iron Hill [27p]. The specimen is no longer fresh, but it is not 
in an advanced stage of decomposition. It was taken as a representative of the 
main sheet near Leadville. 











. C,!l 






















3 46 


njO . -- 



co> . . 





100. 12 



The specific gravity of II was taken at 1G C. By special test in the White 
Porphyry a very small amount of lead was found, = 0.003 per cent, of PbO (Part II, 
Chap. VI). No CO 2 was found in I; that in II, taken together with the increased 
percentage of CaO, indicates the presence of calcite, which is probably an infiltration 
product, as there are dolomite bodies in the neighborhood. The close agreement of 
these analyses is such as might have been expected from the preceding descriptions 
and confirms the views expressed as to the close relationship of the two rocks. 


This porphyry, so called on account of the remarkable amount of pyrite invari- 
ably found disseminated through its mass, owes its importance principally to its sup- 
posed connection with the ore deposits of Leadville. 

Its geographical extent is limited to the district shown upon the map of Leadville 
and vicinity, where it seems to occupy a stratigraphical position, which to the north 
is filled by the Gray and to the east by the Sacramento Porphyry. From the latter it 
is distinguished in field appearance by its almost universally decomposed condition, 
and in its constituents by a relatively small proportion of plagioclase ; from the for- 
mer, in addition, by the absence of large crystals of orthoclase, and from both by the 
want of hornblende. 

As a type, will be taken the unusually fresh rock occurring in White's gulch 
between the Printer Girl and Golden Edge claims [87]. It has a distinct porphy- 
ritic structure, showing numerous white feldspar crystals, with quartz, biotite, and 
pyrite as other recognizable constituents. Altered feldspars are nearly indistinguish- 


able from the white grouudniass, and plagioclase is but seldom ideutiflable with the 
naked eye. There are no large feldspar crystals, as in the Gray Porphyry. Quartz 
occurs most frequently in irregular fragments and rarely contains bays of the ground- 
mass. Biotite appears in distinct leaves, usually altered to a green chloritic substance. 
Through a nearly parallel arrangement of its leaves a stratified appearance is pro- 
duced in some cases. Before disintegration of the rock, the place of the biotite is 
often occupied by ocher derived from the decomposition of pyrite. The latter mineral 
is scattered through the whole rock, but concentrated upon fissure planes by secondary 
processes. Galena appears locally in small quantity, but only on fissure planes. Some 
specimens contain irregular fragments of other rocks, chiefly qnartzites of the Weber 
Grits formation. 

Microscopical No additional origiual constituent is shown by the microscope, with 
the exception of minute crystals of zircon. Apatite, so seldom wanting in rocks of this 
class, has not been identified in the Pyritiferous Porphyry. Pyrite takes the place of 
magnetite and seems to, be an original constituent. Its particles are included iu quartz 
and appear iu arms of the grouudmass, which penetrate or separate quartz grains. 
It is also seen imbedded iu biotite and is scattered through the groundinass iu the 
manner characteristic of the original ore minerals in similar rocks. Few of the feld- 
spars are entirely fresh and most of them are replaced by very fine aggregates of 
muscovite or kaolin. Plagioclase is identifiable iu rare cases and was undoubtedly 
much subordinate to orthoclase in the fresh rock. In the freshest specimen obtained, 
chemical analysis showed 4.62 per cent, of potash and 2.91 per cent, of soda. Quartz 
appears iu angular grains which are sometimes fractured and show parts of but slightly 
difterent optical orientation, separated by thin arms of the groundmass. Fluid inclu- 
sions are abundant in many grains, usually with but little fluid, while empty pores 
are also numerous; but none of glass was seen. Biotite is altered to chlorite or allied 
products, with a separation of yellow needles and tabular crystals, presumably rutile 
and anatase, respectively. 

The groundmass never reaches the coarseness of grain common iu other porphy- 
ries of the region. It is always very finely and eveuly granular, never allowing a dis- 
tinction of quartz and feldspar. 


This type of quartz-porphyry, found in several distinct bodies and exhibiting iu 
all a marked uniformity iu structure aud composition, has been named from its princi- 
pal observed occurrence in the North Mosquito amphitheater [98]. All the bodies are 
dikes in the Archean, and besides the locality mentioned the rock was seen upon the 
north wall of Mount Lincoln [97] and in Cameron amphitheater [96], iu the latter case 
penetrating sedimentary beds. 

It is a light gray rock of flue grain, whose most prominent constituent is quartz 
iu clear, irregular grains, which seldom exceed 0.5 cm iu diameter. Other recognizable 
elements are biotite in small leaves, not abundant, and minute feldspars, which can 
scarcely be distinguished from the light groundmass. A brilliant, black ore in small 
specks is abundant. Glistening hexagonal prisms of what the microscope proves to be 
apatite are often seen, upon close examiuation. 


Microscopical Zircon, ilmenite, pyrite, specular hematite, and probably magnetite 
are present in small quantity, a diversity in such constituents seldom seen in rocks of 
this region. Apatite, noticeable even macroscopically, is developed in stout prisms, 
with many minute inclusions, producing the dusty appearance often described. No 
other rock of the range exhibits a similar development of this mineral. 

Biotite is shown in various stages of decomposition, chlorite being the first product, 
which sometimes gives way to epidote, or, as is clear in many cases, to a micaceous min- 
eral apparently identical with the muscovite which is formed from adjacent orthoclase. 
Accompaniments of this change are yellow needles, presumably rutile, while the iron 
of the chlorite either is carried away or separates out in glistening black ore particles, 
thought to be specular hematite. 

Of the feldspars, orthoclase seems to predominate slightly. Plagioclase is pres- 
ent both in crystals and in the groundmass, where its small microlites are much more 
prominent than usual. Quartz is regularly but rather sparingly present in large 
grains, seldom showing crystal outline and containing numerous small fluid inclusions, 
while none of glass was observed. A microcrystalline, granular mixture of quartz 
and two feldspars, with but very little primary mica, makes up the groundmass. 

Chemical analysis shows 68.01 per cent, of silica, 4.36 per cent, of potash, and 
4.26 per cent, of soda. The alkalies are rather more nearly balanced than one would 
suppose them to be from the microsc >pical examination. 


This rock is the most important of the varieties belonging to the second division 
of the quartz-porphyries of the district, namely, those in which the porphyritic structure 
is macroscopically very plain. It has been called the Lincoln Porphyry from the fact 
that it is best developed in and about the mass of Mount Lincoln, forming the extreme 
summit of that peak, and in this once important mining district bearing approximately 
the same relation to the ore deposits which near Leadville is assumed by the White 
Porphyry. As will be shown later, it is very closely allied to the Leadville Gray Por- 
phyry and has intimate connection with the Eagle River Porphyry and other rocks 
of the adjoining district upon the north. In the following description will be con- 
densed the observations upon twenty specimens collected at different places. Devi 
ations from the type rock of Mount Lincoln will be specially noted. 

Macroscopicai. The essential constituents are quartz, orthoclase, plagioclase, and 
biotite, all occurring in distinct crystals and imbedded in a compact groundmass of 
varying importance. A part of the orthoclase appears in large, stout crystals, fre- 
quently two inches in length, usually pinkish in color, and so fresh and glassy as to 
resemble markedly the sanidine of younger rocks. They are often Carlsbad twins 
and contain noticeable inclusions of biotite leaves. For most occurrences of the por- 
phyry these large orthoclase crystals are eminently characteristic, though their (level 
oprnent has been hindered in some cases, particularly in dikes and small masses. In 
some of these instances small crystals of pinkish color are plainly more numerous- 


than in the type rock, but in others they cannot be well distinguished from the tri- 
clinic feldspar. Plagioclase is always very abundant in white individuals, seemingly 
less fresh than the orthoclase, although a striatiou can often be seen on the basal 
cleavage surfaces. Biotite occurs in small hexagonal leaves, which are sparingly but 
uniformly scattered through the whole. They are seldom fresh and usually appear to 
be changed into a green chloritic mineral. The quartz appears as a prominent niacro- 
scopical constituent, showing, as a rule, a development of pyramidal planes, to which 
the prism is occasionally added. 1 The groundmass is dense and homogeneous in appear- 
ance, usually grayish in color in fresh rocks, and very distinct. Only occasionally does 
it become subordinate. Ore particles are plainly distinguishable in it. 

Specimens of the rock obtained from exposed surfaces of high mountains are 
usually bleached and light-gray in color, slightly stained by hydrous oxide of iron, 
while in tunnels and mine workings the rock is generally greenish through the chlo- 
ritic decomposition products of the biotite. 

Microscopical The microscopical examination reveals the following as original 
constituents, named in order of formation, viz : Allanite, zircon, magnetite, titanite, 
apatite, biotite, plagiodase, orthoclase, and quartz. All the minerals named occur in 
more or less perfect crystal form and are imbedded in a granular groundmass, consist- 
ing of plagioclase, orthoclase, and quartz. The amount of plagioclase in the ground- 
mass is doubtless small, for it is so abundant in the form of imbedded crystals that 
but little substance could have remained for the second generation. The size of the 
grains in the groundmass is so small that one cannot well distinguish between quartz 
and orthoclase, but the holocrystalline nature is evident. No microfelsitic or glassy 
matter has been found in any rock of this type. 

Of the accessory constituents the most noteworthy is allanite, which appears 
very sparingly but constantly in this and other rocks of the Mosquito Range and 
adjoining regions. It is apparently the first mineral formed, or is perhaps contempo- 
raneous with zircon, these two minerals penetrating even magnetite and apatite. 
During the first study of these rocks the nature of this mineral was not determined, 
but, through the subsequent detailed investigation of a similar porphyry of the Ten- 
Mile mining district, enough was isolated by means of the Thoulet solution to allow of 
chemical analysis. The analysis, made by W. F. Hillebrand, was not completed, owing 
to accident, but it established the presence of Ce and La with the absence of Di, while 
Fe 2 O 3 and SiO 2 were the remaining constituents of note. At about the same time Mr. 
Joseph P. Iddings, of the TJ. S. Geological Survey, determined the same mineral 
crystallographically in various rocks of the Great Basin in Nevada. As a rule 
the allanite is seldom macroscopically visible in the rocks of the Mosquito Range, 
while it is quite noticeable in those of the Ten-Mile region. It appears in small 
prisms of maximum length of about 5 mm , has a brilliant dark resinous luster, and 
when decomposed stains the surrounding zone in reddish-brown shades. The chance 
sections show a transparent, yellowish-brown mineral, with no. distinct cleavage. 
The faces developed seem probably referable to oc P, co Pee , OP, and + Poo . It 
is often twinned, possibly parallel coP* , as by epidote, and in several sections which 

1 On the ridge east of Hoosier pass the outcrop of a porphyry sheet is marked by quartz crystals 
which have weathered out of the underlying rock and which show both pyramid and prism. 


seemed to lie approximately parallel to oo P <x extinction took place at 35 to 38 from 
the vertical axis. Pleocbroism distinct, the color varying from light to dark shades 
of yellowish brown. 

Zircon is abundant in minute clear crystals. Fig. 3, Plate XXI, shows two zircon 
crystals of characteristic form included in a quartz grain of a Lincoln Porphyry. 
Titanite was seen in but one or two specimens, and then very sparingly. Magnetite 
and apatite occur as usual in such rocks. Biotite frequently includes apatite and zir- 
con and may be penetrated by allanite. It is otherwise interesting from its altera- 
tion products, which will be discussed below. 

The plagioclase, which is so prominently developed in crystals, is probably an 
oligoclase, judging from the extinction in the zone perpendicular to the lamina?, the 
direction being always within the limits of oligoclase. Orthoclase is seldom met with 
among the crystals of medium size, being present in larger individuals or in the irreg- 
ular grains of the grouudmass, where it presents nothing noteworthy. The signifi- 
cance of this development is poiuted out later. 

The large quartz grains and crystals contain a few fluid inclusions of irregular 
shape, and bays of granular grouudmass penetrate them without any very marked 
change iu texture of the mass. Glass inclusions are very rare in any specimens of the 
Lincoln Porphyry and never have been noticed in the type rock of Mount Lincoln. 
Quartz crystals have frequently exerted an influence upon grains of the same mineral 
in the adjoining grouudmass, which have within a narrow zone the same optical orien- 
tation as the crystal. There is 110 regular relation of the quartz to the orthoclase within 
this zone. 

Alteration. Biotite is usually more or less altered and presents different products 
under different circumstances. In a specimen from the head of Clinton gulch, Summit 
County, the chief product is a micaceous mineral, seemingly muscovite, which con- 
tains numerous needles of rutile. In other cases chlorite is first formed, and this is 
also accompanied by yellowish needles, or by irregular paler grains of undeterminable 
nature, which resemble titauite or at times anatase. Epidote seems to replace the 
chlorite, or in other cases to come directly from the biotite without any intermediate 
stage. The feldspars give place to an aggregate of muscovite leaves in most cases, but 
calcite is frequently seen as a product from plagioclase and epidote, also, may be often 
found resulting from the alteration of the tricliuic feldspar. As in some of the other 
types to be described epidote is very commonly a result of alteration of pure feldspar, 
there appears no good reason for regarding it as induced by the presence of assumed 
inclusions in the case of the Lincoln Porphyry. Secondary chlorite is sometimes 
deposited throughout the groundmass, giving a green color to the rock. 


This rock, which occurs in the vicinity of Leadville, is the nearest relative of the 
Lincoln type. It is, however, directly connected with a porphyry which has its chief 
vent of eruption and largest masses in the adjoining region to the north, at the head- 
waters of the Eagle River This latter type will be fully treated in the report upon 
the geology of the Ten-Mile district, aud, as other allied rocks can there be drawn into 
the discussion, the present description will not go deeply into a comparison of types. 


The Gray Porphyry is seldom fresh, as it occurs in the region adjacent to the 
ore deposits, where agencies of alteration have been active, and presents usually a 
greenish-gray rock, showing numerous crystals imbedded in a prominent grounduiass. 
The minerals are the same as those of the Lincoln Porphyry, viz. large orthoclase, 
small and numerous plagioclase, and biotite crystals. In the mines the rock is so 
bleached that even jvith its original large crystals, it is not easily distinguished from 
the White Porphyry. The quartz contains large bays or penetrating arms of the 

Microscopical. One never-failing and striking peculiarity of this, in distinction 
to the Lincoln type, is the presence of outlines of a former constituent of the rock, which 
would seem to belong to hornblende, although no trace of that mineral in fresh condi 
tion could be found. These outlines are usually marked by dark grains, and inclose 
a fine grained, grayish decomposition product, which acts very feebly in polarized 
light. They are not wanting in any slide examined, and are always of the same 
appearance, even when other minerals are entirely fresh. 

The feldspars of the Gray Porphyry, unlike those of the Lincoln Porphyry, con- 
tain numerous fluid inclusions, which are generally arranged parallel to the chief cleav- 
age planes. Besides these, there are many irregular interpositions, either devitrifled 
glass inclusions or portions of the groundmass in a less crystalline state than it now 
presents in the main mass of the rock, They are light reddish-brown in color, and plen- 
tiful in most of the small crystals. Distinct glass inclusions, although not noticed in 
any feldspars, are very characteristic of the quartz grains. They are often sharply 
negative crystalline in form, and sometimes show devitrification; others are spherical, 
and in these it can often be seen that from opposite poles, which probably lie in the 
vertical axis of the quartz grains, cracks penetrate the sphere in three planes, cutting 
each other at about 60. If the sphere be cut by the section at right angles to the 
axis uniting these poles and near one of them, there results a delicate six-armed figure, 
which appears as if contained in the quartz itself. The groundmass, though holocrys- 
talline, is much finer-grained than that of the Lincoln Porphyry, and shows a tendency 
to an irregular intergrowth of quartz and feldspar. 

Occurrence. Gray Porphyry is quite limited in distribution, being confined to 
the immediate vicinity of Leadville, and to the region northwest of that point. As 
.has been described in detail (p. 80), it occurs chiefly in one large sheet, with numer- 
ous offshoots, and the large sheet has been directly traced to a connection with an 
immense body at the headwaters of the Eagle River. The hornblende of the Gray 
Porphyry is considered analogous to the crystals of that mineral observed in small 
dikes which are offshoots from the Eagle River mass. 

Chemical composition of the Lincoln and Gray Porphyries The following rock analyses 

were made by W. F. Hillebrand. 

I is of Lincoln Porphyry, summit of Mount Lincoln [75]. It is quite fresh in 
appearance, although showing some muscovite, calcite, and chlorite 3 when examined 



II is of Gray Porphyry, Quota shaft, Johnson gulch, near Leadville [59a], 
fresh appearing, but somewhat altered, with the same products as in the former rock. 




66 45 

68 10 





15 84 

14 97 


2 59 


FeO . . 

1 43 

1 10 





> 90 

3 04 





1 21 

1 10 


2 89 

> 93 


3 92 

3 4tl 

LljO .. .. 






1 35 


PiO . . . 







100 09 

100 11 

Specific gravity, 10 C .... 



The relative amounts of soda and potash indicate an abundant soda-liine feldspar. 
The titanic oxide found corresponds to the suggestion that the yellow needles in the 
decomposed biotite are rutile, for the magnetite does not give signs of an intermixture 
of titanic iron through its alteration products. The presence of strontia in determiua- 
ble quantities is unusual and worthy of note; it doubtless comes from the plagiochue. 
Instances of its determination in rocks are rare, 1 though it would probably be found 
in many cases if sought for. 

Although the large pink or white orthoclase crystals are characteristic of most 
of the occurrences referred to the Lincoln and Gray Porphyries, still a number of cases 
were found where the rock seemed identical with these types in every respect, except- 
ing that the large crystals were wanting. In some bodies of rock, moreover, the large 
crystals were by no means equally distributed. It seemed therefore desirable to ascer- 
tain more definitely the source of the alkalies in the rocks analyzed. In each case 
enough of the large orthoclase crystals had been included in the material used for 
analysis to give average results. 

In the mass of Mount Lincoln a dike of rock was found which was considered 
as a representative of the Lincoln Porphyry [78], although it was darker, more com- 
pact, and contained none of the large pink orthoclase crystals. Alkali determinations 
gave 2.42 per cent, of potash and 3.15 per cent, of soda, very nearly the same ratio as 
in the type rock. There was also found 64.10 per cent, of silica. The reduced amounts 
of all these are doubtless due to the increased quantity of biotite and of ore in this 
dike rock. 

In the next place the Gray Porphyry, of which the complete analysis had been 
made, was subjected to further investigation. Alkali determinations were made in the 

1 Streng, Neues Jahrbucli fur Mineralogio, etc., p. 537, 1867. 



mass of the rock, carefully avoiding the large pink crystals, with the result of 2.95 
per cent, of potash aiid 2.61 per cent, of soda. As small flakes were used for this 
purpose, it is probable that the grouiiduiass was present in abnormal quantity, thus 
causing a relative increase in potash, even while excluding the large orthoclase crys- 
tals. Plagioclase was found to be much subordinate in the grouudmass, as stated above. 
The large pink orthoclase crystals themselves were then analyzed, with the result: 


62 22 


'20 33 



K 2 O 

8. 31 

Na. 3 O 


Li 3 O 


Tom . . 


Loss - - 


100. 00 

Careful examination of the material used showed only a few specks of biotite, but 
some soda-lime feldspar must have been present, judging from the large amount of 
lime found. A determination in another clear crystal chosen for its apparent purity 
gave nearly 3 per ceut. of lime again. The loss is thought by the analyst, Mr. Hille- 
brand, to be chiefly soda. 


Of the distinctly plagioclastic rocks of the region but very few are granular in 
structure, the great majority being diorite-porphyries, or porphyrites, as they will here- 
after be designated. The three granular diorites fouud, represent three very distinct 
varieties, one of them being the only pyroxene-bearing rock occurring within the area 
of the map. All occur, moreover, in the same gulch, and quite near each other. 


This rock occurs on the south side of Buckskin gulch, Park County, as a broad 
dike, forming for some distance the southeast wall of one of the elevated amphitheaters 
on Loveland Hill, and thence projecting as a knoll into the gulch opposite the Ited 
amphitheater. It disappears under loose material before reaching the stream bed, and 
no continuation of the dike on the north side of the gulch was observed. The rock 
has a fine, evenly-grained structure, with feldspar and quartz strongly predominat- 
ing over the small, irregular leaves of biotite. 

Microscopical examination shows zircon, magnetite, apatite, biotite, plagioclase, 
orthoclase, and quartz as original constituents. Xone of the essential minerals is 
well developed in crystal form and none shows noteworthy peculiarities. Plagioclase 
is largely in excess over the orthoclase and quartz is quite abundant. All are quite 
fresh, the biotite alone showing incipient decomposition [117]. 


A broad dike crossing the head of Buckskin gulch from Democrat Mountain in 
a nearly east-and-west direction was found to consist of a very simple, normal diorite 


[116]. It is fine-grained, yet shows distinctly to the naked eye all its prominent con- 
stituents. Feldspar, a large part of which is clearly plagioclase, subordinate quartz, 
hornblende in prisms with occasional terminations, a little brown biotite, yellow titan- 
ite, and dark ore grains are all easily recognized. The microscope shows zircon and 
apatite in addition, while chlorite and epidote are seen to result from the alteration of 
both biotite and > hornblende. Muscovite forms in the orthoclase, which here seems 
,much more attacked than the plagioclase. There is no groundmass and of the 
essential constituents only hornblende is developed in crystal form. 

A very similar dicrite was obtained from a prospect tunnel in French gulch, Lake 
County [115], in which pyrite replaces magnetite as the ore and zircon and titanite are 
very abundantly developed. Biotite and quartz are even less prominent than in the 
preceding rock. 


In the Bed amphitheater, on the northeast side of Buckskin gulch, there occurs a 
dike of a darker, finer-grained diorite than either of the preceding types [118]. Horn- 
blende, biotite, plagioclase, and a little quartz may be macroscopically detected. The 
microscope shows zircon, titanite, magnetite, hematite, apatite, biotite, augite, horn- 
blende, plagioclase, orthoclase, and quartz. Augite appears most abundantly in the 
freshest specimen, and certainly undergoes alteration to green hornblende, which, 
though not fibrous, like typical uralite, is still by no means so compact as the common 
dioritic hornblende. It is not possible, from the specimens examined, to say with cer- 
tainty that any of the hornblende is original, although the association of the minerals 
in the freshest specimens is such as to indicate a contemporaneous .formation of bio- 
tite, augite, and hornblende. The latter two occur in irregular grains and the augite 
has none of the pinkish tinge common to it when appearing in diorite. This rock is 
remarkable as the only eruptive of the district in which augite has been found. 

Plagioclase appears abundantly in small grains, while orthoclase and quartz form 
the cementing material. Chlorite and epidote result from the alteration of hornblende 
and biotite; muscovite and calcite, from the feldspars. 


Under this heading will be discussed a large number of distinct occurrences, 
which, unlike those of the quartz-porphyries, belong for the most part to small rock 
masses. There are in this group no markedly prevailing types to which the difl'ereut 
rocks can be assigned, and the chief interest here lies in noting the great variations 
possible, both in structure and composition, in what are practically equivalent masses. 
One distinction, however, is feasible, viz, that between a variable subgroup, in which 
a triclinic feldspar is evidently strongly predominant, and a few rocks occurring in 
larger masses, in which orthoclase is also prominent and which seem at first glance 
more nearly related to the quartz-porphyries than to the marked plagioclase rocks of 
the first division. These latter types are referred to in the main report as quartz- 
porphyries, and are so represented upon the map. They are called by the local names 
Sacramento Porphyry and Silverheels Porphyry. Later investigation has shown 
them to be plagioclastic rocks, and as such they will here be treated. In describing 
them the general and variable group will first be considered and then the local types. 



The characteristic primary constituents of these rocks are the minerals zircon, 
allanite, apatite, magnetite, biotite, hornblende, plagioclase, orthoclase, and quartz. 
To these, as occasional accessories, may be added ilmenite and pyrite. All the common 
non-essential elements are developed in the ordinary way, and none is so abundant 
or so rare as to deserve comment. Allanite is not always present in the thin sections 
examined, but its observed distribution among different types is such as to warrant 
the belief that it is sporadically present in all the rock masses of this group. 

Feldspars All crystals of the first period of consolidation which have been 
identified are plagioclase, with but one possible exception, referred to later (p. 339). 
Orthoclase may be sparingly developed in this way in a few cases, but the freshness 
of the plagioclase in nearly all specimens collected and the ease with which the stria- 
tion can be seen upon the basal cleavage plane make it certain that a monoclinic 
feldspar must be very rare. In the groundmass, on the other hand, plagioclase is not 
visibly present at all in many cases, while orthoclase is very abundant. 

- The plagioclase crystals are small, white, stout in form, and correspond exactly 
to those described in the quartz porphyries. They are chemically near oligoclase, 
judging from the optical properties, for the maximum observed extinction in the zone 
perpendicular to the usual twinning plane is but 20. In a number of crystals twin- 
ning according to the Carlsbad law is apparently combined with that of the albite 
law, as, for example, in one section, falling at right angles to the bracbypinacoid, there 
are 20 laminae, of which five pairs extinguish sharply at 8 45', the other five pairs at 
from the twinning plane. In a few cases more than two directions of extinction 
were noticed in sections apparently lying in the macrodiagonal zone. In one crystal 
laminae were found extinguishing at 1, 4 30', 8, 13, and 20, several pairs showing 
the last two values. A satisfactory explanation for this action has not been found. 
It may be that laminae of different feldspars are hereintergrown, but such a conclusion 
must be supported by further data than are here available. 

A delicate zonal structure is occasionally seen in plagioclase crystals, but the 
slightly varying angles of extinction do not indicate any pronounced changes in 
basicity of the different zones. 

Biotite Biotite appears as a constituent iti three distinct forms: as macroscopic 
hexagonal leaves, in aggregates of small irregular flakes, and as minute leaflets in the 
groundmass. The large leaves are brown when fresh and often exhibit ragged edges 
when seen tinder the microscope, caused by the attachment of many flakes correspond- 
ing to those in the groundmass. Allanite, zircon, magnetite, and apatite penetrate 
the larger leaves. The tiny leaflets which enter at times richly into the composi- 
tion of the groundmass are irregular in shape and rarely over 0.03 mm in diameter, 
sometimes sinking to a minuteness requiring the highest power of the microscope to 
resolve them into separate flakes. They are greenish in color and at first glance it is 
not easy to discover t'heir nature as mica; but their marked pleochroisin and strong 
absorption in proper position renders their character certain. These flakes of green 
mica are often arranged one after another, partially overlapping, making needle-like 
aggregations, easily mistaken for hornblende with a low magnifying power. 

Hornblende The hornblende is compact, of a green color in ordinary light, and 
generally presents quite well-defined crystals, the faces ccP, ccP do, OP, and P being 


ofteu visible on macroscopic crystals. It occurs either as a macroscopic clement of the 
rock, in the form of minute needles in the grouudmass, or, lastly, in clusters of small 
irregular individuals, and then usually associated with biotite leaves. 

The small needles are sometimes well terminated (see Fig. 3, Plate XX). Still 
it is the rule to find the ends irregular, while the prism is sharply defined (see Fig. 4, 
Plate XX). The pleochroism is well marked, and the maximal angle of extinction in 
the prismatic zone is nearly 18, measured from the vertical axis. Twinning parallel 
to oo P a, is common and is frequently polysyuthetic. 

The hornblende occasionally includes crystals of apatite, magnetite, rarely biotite, 
and clear microlites of zircon. It is commonly very fresh, and when decomposition 
has begun the first product is usually chlorite, from which epidote is formed. 

Quartz. There are but few rocks of this kind in which quartz is prominent as a 
macroscopic constituent. In some of these, usually the more acid ones, it forms well- 
defined crystals, but it is more common to see it in rounded grains, seemingly quite 
variable in quantity, in occurrences which are otherwise nearly identical. These 
rounded particles undoubtedly represent partially remelled crystals of the first gen- 
eration, and their variability is here not remarkable. The chief development of the 
quartz is, as perfectly natural, in the grouudmass, with orthoclase. Inclusions are not 
abundant in any of the crystals, though all earlier minerals do penetrate it in observed 
instances. Glass inclusions have never been found and those with fluid contents are 
rare. The groundmass seldom penetrates the large crystals. 

Groundmass The groundmass of those porphyrites which contain hornblende 
and biotite mainly as macroscopic elements is very uniform in constitution and struct- 
ure. It consists of an evenly granular mixture of quartz and feldspar, with small 
octahedrons of magnetite scattered through it. The feldspar is seldom definitely 
determinable as such, but its presence is inferred from a formation of muscovite, 
where the rock is much altered, and because the quartz grains, through their stronger 
polarization, stand out in contrast to the rest of the colorless grouudmass. By far the 
greater part of this feldspar is niouoclinic, for plagioclase was observed to enter into 
the composition of the groundmass in but few cases, and then in the form of thin 
plates, quite distinct from the irregular grains of orthoclase. The average size of 
the grains of quartz and orthoclase is 0.02 min , so that a complete separation of 
these minerals is never possible. There is never a trace of microfelsitic or glassy sub- 
stance, and only in contact specimens is the greater part of the groundmass crypto- 
crystalline. As has been mentioned above, biotite and hornblende enter into the con- 
stitution of the groundmass in very varying quantities, and only when present in 
great abundance do they render the mosaic of quartz and feldspar obscure. The 
quartz has a tendency to develop in clusters of irregular clear grains in certain cases. 

The distinguishing peculiarity of a certain minor subgroup lies chiefly in the 
character of the groundmass. This consists principally of an iutergrowth of quartz 
and orthoclase, according to no discernible law, now the quartz, now the orthoclase 
being the inclosing mineral, and their relation is only made clear between crossed 
nicols, when it is seen that within the limits of certain irregular patches all the quartz 
and all the orthoclase has each its own optical orientation. The outline of the inclos- 
ing mineral has no relation to crystal form, and this iutergrowth nets throughout like 
the ordinary grouudmass, filling the interstices between the large crystals. The macro- 



Fig. I 

Fid. 2 

Fig. 3 



scopical effect of this structure is to reuder the groundmass much less distinct in con- 
trust with the crystals than is the case in the types of the main group. Flakes of 
biotite, grains of magnetite, &c., are scattered about in this groundmass with the 
same irregularity as in any other. 

A tendency to a micrographic-granite structure was noticed in two of the porphy- 
rites. It seems to have been induced by the presence of the rounded quartz grains 
above described. Each of these is surrounded by a zone in which quartz and 
orthoclase are more or less regularly iutergrown. The appearance, as seen' under the 
microscope, is that of alternate fibers of quartz and orthoclase, with a more or less 
distinct radiate arrangement about the large quartz grain, all the quartz substance, 
in both granule and groundmass, having the same optical orientation. A similar phe- 
nomenon was not observed in connection with large particles of feldspars, and those 
portions of the groundmass showing a regular intergrowth apparently independent 
of any crystal may have been formerly related to a quartz grain situated just above 
or just below the plane of the present section. 

In such a thoroughly crystalline rock a fluidal structure can only be expressed 
by the position of the hornblende needles or biotite leaves with reference to the large 
crystals. Such a relation is often observed, and it is also not rare to find hornblende 
crystals broken and biotite leaves folded and crumpled, attesting to movements in the 
partially solid rock. 

Structural forms The greater number of the rocks observed form a continuous 
series whose extremes are very dissimilar, and the relationship can be most easily 
understood and explained with the help of the subjoined table : 



Macroscopic development. 

In groundmass. 

f Biotite 


y e>* 

c Hornblende . . . 


e Biotite 


( Hornblende. . . . 
e Biotite 

la numerous crystals 

Entirely wanting. 



IV ... 

' Hornblende . . 
< Biotite 

Sligbtly predominant 

Few small needles. 


( Hornblende . . . 

r Biotite 


( Hornblende ... 

Rare or wanting 


c Biotite 

VII .. 

( Hornblende 

Very abundant 

Very abundant. 

Under Division A are included rocks with a light, homogeneous-appearing 
groundmass, containing no microscopic individuals of the basic mineral which is so 
prominent in macroscopic crystals, this in the one case (I) being biotite, in the other 
(II) hornblende. Under 0, at the opposite structural extreme, where the groundmass 
is filled with minute flakes or needles of a dark mineral, are also two modifications, 
one a biotite (VI) (Fig. 1, Plate XX), the other prevailingly a hornblende rock (Fig. 
2, Plate XX). These are both dark and compact, showing comparatively few macro- 



scopic elements, standing in marked contrast to those forms under Division A. Between 
these extremes, in regard to botli structure and composition, are the forms embraced 
under B. In these the groundiuass contains more or less of one or both of the dark 
basic minerals, and in proportion as these minerals enter into the composition of the 
groundmass the macroscopic elements become less distinct, thus forming a gradual 
transition to the Division C. 

Division A The plagioclase usually stands out very plainly in these rocks, and it 
is evident that no orthoclase is present in macroscopic individuals. Quartz occurs in 
good crystals and rather plentifully. The groundmass is microcrystalline and pos- 
sesses a very regular granular structure, its components being almost exclusively quartz 
and orthoclase. A dike in gneiss, near a little lake northwest of Mount Lincoln, rep- 
resents the typical hornblendic variety [1120], while a similar dike in North Mosquito 
amphitheater is of the corresponding biotite rock [119]. Several occurrences at the 
head of Buckskin gulch are nearly allied to these type rocks. 

Division B By far the larger number of the porphyrites in the series fall within 
this division. In the three subdivisions of the table, one or botli of the heavier sili- 
cates appear in the groundmass as well as in larger crystals. If the groundmass 
minerals are regarded as belonging to a second phase of the rock's existence, one 
of the striking peculiarities of this division is most natural, while from another point 
of view it might seem strange. The peculiarity referred to is the observed indepen- 
dence of the daik basic silicates occurring in the groundniass, of the species which 
may be developed as macroscopic constituents. The formation of hornblende in 
numerous large crystals during the first period of consolidation does not necessarily 
demand that the same mineral should be developed in the second period. The changed 
conditions attending the final consolidation may produce biotite or hornblende, or both 
of them, uninfluenced, or at least uncontrolled, by the earlier crystallizations. The 
table above shows this, but a study of the variations in the different rocks collected 
makes the fact much plainer. The rock most frequently met with in all the district 
belongs to Type V of this division. It is the one found in the intrusive sheets on the 
sides of Mosquito [127] and Buckskin gulches [12fi], on Mount Lincoln, or in dikes in 
the Archean, as on Bartlett Mountain [124] and Democrat Mountain [2.19]. The lower 
figure of Plate VII, page 84, shows the macroscopic a 1 appearance of this rock very 
well. Hornblende crystals are frequently well terminated in this modification, and 
owing to the minute size of many well-shaped prisms, while all intermediate stages 
arc also represented, it becomes difficult to decide whether there has or has not been a re- 
currence in the formation of hornblende prisms with good crystal form. Fig. 3, Plate 
XX, was designed to show both large and small prisms of hornblende with good ter- 
minal planes, but the imperfect execution of the prints leaves much to the imagination. 
In Fig. 4 of the same plate are shown needles of hornblende with the more common, 
irregular terminations. 

Division C The compact rocks of this division are not very numerous. The two 
occurrences illustrating best the micaceous and hornhlendic varieties occur together 
in North Mosquito amphitheater. One of these, the biotite rock [260], was analyzed 
(p. 340), and its micro-structure is indicated by Fig. 1, Plate XX. Two other compact 
rocks deserve special mention under the next heading. 


The Arkansas Dike The long straight dike at the head of the Arkansas presents 
some remarkable phenomena, which cannot be explained satisfactorily from the data 
collected in the one short visit made to that area. It is a special matter for regret that 
no time for further examination could be taken. This dike consists of what are re- 
garded as two eruptions of the same magma. The older rock is fine-grained and ex- 
hibits a few small feldspars and biotite leaves as sole recognizable macroscopic con- 
stituents [130]. The younger rock [129], which cuts irregularly through the former, 
now on one side, now on the other, or even running along the center, is also very dark 
and compact in the main, but is sharply distinguished by numerous large quartz 
crystals and by worn aud well-rounded fragments of slightly pinkish orthoclase. 

A heliotype representation of this curious rock is given in the upper figure, Plate 
VII, natural size. These orthoclase fragments are all like pebbles, showing no trace 
of sharp angles. They reach a maximum observed diameter of over 5 cm , and none 
was noticed of less than l cm . While never glassy, they seem quite fresh, represent 
but one crystallographic individual each, aud are in no way related to anything seen 
in other occurrences of the porphyrites. The quartz crystals reach a diameter of over 
l cm in this rock and are always quite well-defined in crystalline form. Hornblende 
takes a prominent place beside the biotite in the microscopical constitution. It is a 
curious fact, commented upon later, that in spite of its large quartz crystals the 
younger porphyrite has but 59.26 per cent. SiO 2 , while the dark, compact, older rock 
contains G0.29 per cent. Repeated determinations for both rocks show similar results. 

The origin of these orthoclase pebbles is very problematic. To consider them as 
earlier secretions of the porphyrite magma is to assume conditions to which no other 
rocks of the group have been subjected, judging from the total absence of such orthoclase 
in them. Inclusions of basic microlites would seem to be almost inevitable, if these 
orthoclase individuals had formed in the midst of the minerals which one must sup- 
pose to have reached consolidation before them. A thin section of one of these pebble- 
like fragments shows that the feldspar is common orthoclase aud that it contains 
inclusions of magnetite, biotite, aud quartz. No zircon, allauite, or apatite was seen. 
The included quartz grains are small and crowded with fluid inclusions, in many of 
which the bubble is in active motion. 

From the above considerations it is difficult to reach a conclusion as to the origin 
of these feldspar masses. The absence of zircon and apatite and the presence of 
quartz with such numerous fluid inclusions seem to indicate that the mineral is not 
an earlier secretion of the porphyrite magma. The fact that, while rounded fragments 
of gneiss and granite are abundant in many dikes in the Archeau, they were not 
found accompanying these pebbles, would seem to throw doubt upon the accidental 
nature of the fragments in question. Until a more thorough examination of the occur- 
rence can be made no satisfactory conclusion seems possible. 

Chemical composition The analyses given below were made by W. P. Hillebrand. 
Under I is given the composition of the typical horublendic variety, occurring in thin 
intrusive sheets on the sides of Buckskin and Mosquito gulches. It is Type V of the 
table above; its macroscopical appearance is shown in Plate VII, lower figure, and its 
microstructure in Plate XX, Fig. 3. The specimen analyzed came from the Northern 
Light claim, in lower Buckskin gulch [126]. The rock as a whole is quite fresh, 
although the few biotite leaves and occasionally a hornblende crystal are more or less 



decomposed, chlorite and calcite being the chief products. The plagioclase is still 
quite fresh, but some filmy calcite is scattered through the groundmass. Apatite is 
rather rare in this rock. 

Analysis II was made upon a compact biotite lock, Type VI of the table, from 
North Mosquito amphitheater, where it occurs as a dike in gneiss [260]. There is uo 
hornblende present in this rock and biotite appears mainly in numberless minute, 
greenish flakes. Quartz is abundant in clusters of small grains in the grouudmass, 
but seldom reaches macroscopic dimensions. Apatite is quite abundant. Pyrite is 
the chief ore of the rock, accompanied by some magnetite. Except for some calcite 
and chlorite the rock seems to be very fresh. 









1C 74 

15 73 



1 68 

FeO . . 

3 27 






7 39 

4 22 




4 08 




1 43 



3 98 













0. 90 0. 48 8. 




Specific gravity 1G C . 

^ TO.- 


Discussion of analyses There is far more difference between the two rocks than 
one would surmise from the microscopical study, but it is after all not so remarkable 
when one considers how little substance is actually represented by the minute flakes 
of biotite in contrast to that in the numerous prisms of hornblende. The difference lies 
chiefly in the large amount of hornblende, while the feldspars in both are plainly soda- 
linie-hearing varieties to a very large extent. 

In order to test the influence of the amount of hornblende present, as indicated 
by the macroscopical appearance of the rock, special silica determinations were made 
by Mr. Hillebrand upon various types. First, two rocks having closely the habit of 
that furnishing Analysis I, but occurring as dikes in the Archenu, one in Ten-Mile 
amphitheater [125], the other on the east wall of Arkansas amphitheater [131], were 
tested, and yielded, respectively, 57.76 per cent, and 57.33 per cent. SiO*, ligim-s 
agreeing quite closely with those of the analysis. Secondly, the compact horubleudic 
rock from North Mosquito amphitheater [132], which occurs side by side witli the cor- 
responding biotite rock (Analysis II), was found to contain 54.54 per cent. SiO 2 . Thirdly, 
a rock from the extreme head of Buckskin gulch [121], which contained very little 
hornblende in the groundmass and had a number of the rounded quartz grains macro- 


scopieally visible, proved to carry 65.73 per cent. SiO 2 . The range in silica is thus 
more than 10 per cent., and it is chiefly affected by the amount of hornblende present 
in the rock. 


This rock, which was at first classed as a quartz-porphyry and is so represented 
on the map, occurs in a large mass at the head of Big Sacramento gulch, whence intru- 
sive bodies extend to the north and to the southeast. 

Description In structure this rock resembles those modifications of the Lincoln 
Porphyry in which the formation of the large orthoclase crystals has been hindered. 
It shows many white plagioclase crystals of the usual stout habit and a number of 
smaller, less distinct individuals which are less fresh, most of them being ortho- 
clase. Both biotite and hornblende are present in distinct individuals, and quartz 
occurs in round grains, which are not very plentiful. The groundmass containing 
these elements is subordinate but yet distinct. It contains pyrito and magnetite, and 
a sufficient number of small biotite leaves to be dark-gray in color, when fresh. Chlo- 
ritic decomposition products render it darker in most specimens collected. Epidote, 
which is often prominent in more or less altered specimens, will be spoken of below. 

With the microscope it is found that zircon, allanite, .apatite, and titanic iron, 
the last recognized by cleavage and alteration products, are, further components of 
the rock. Allanite is not so plentiful as in some other types described, but it is ob- 
served in one or two slides and is probably a regular but sparsely distributed con- 

The microscope shows that plagioclase is developed almost exclusively in the 
form of porphyritic crystals and that but few of these are certainly orthoclase, 
although the latter mineral forms with quartz nearly the entire groundraass, the dark 
silicates seldom appearing as constituent particles in this later product of consolida- 
tion. Orthoclase is usually more decomposed than plagioclase, being cloudy after the 
manner commonly seen in much older rocks. The majority of the plagioclase crystals 
arc clear in the center, but show incipient decomposition in the. outer zones. The 
laminae composing them are either broad or very narrow and the maximum angle of 
extinction in the macrodiagonal zone is 20, indicating that varieties more basic than 
oligoclase are rare if present at all. 

Hornblende and biotite have a thoroughly normal appearance, and are only 
interesting through their decomposition products, to be considered below. Inclusions 
of the early accessory constituents are a matter of course in all the large crystals, but 
they are never abundant and are never accompanied by glass, so far as observed. 
Fluid inclusions occur sparingly in quartz and feldspar. 

Decomposition products Noteworthy facts concerning the decomposition of rock- 
building minerals may be observed in the Sacramento Porphyrite. The specimens 
collected are divisible into two classes, the one showing a bleached rock, the other 
containing macroscopically developed epidote. In the latter rocks [83-85] the micro- 
scope shows a more or less marked tendency to the formation of epidote from both 
feldspars, as well as from biotite and hornblende. In the last two minerals a dark, 
strongly pleochroic chlorite is the forerunner of the epidote, while in the feldspar no 
intermediate stage of any kind can be detected. Muscovite and calcitc, the common 
products of alteration in feldspars, are here but slightly developed. 


In the second class mentioned tbe processes of decomposition have produced a 
light-colored rock, in which the biotite is replaced by a light, straw-colored substance 
with silvery luster, while hornblende and ore particles have almost entirely disap- 
peared. The microscope shows that decomposition has from the beginning taken an 
entirely different course from that just described, although here, as there, the tendency 
has been to the formation of a particular mineral, that mineral being inuscovite instead 
of epidote. Muscovite resulting from the decomposition of biotite has been described 
(p. 324) in the case of the Mount Zion Porphyry, and the present instance is very simi- 
lar The muscovite is filled with minute, pale yellowish needles and grains (rutile ?}, 
which cause the macroscopically visible tinge of color. That this mineral is really 
muscovite it may be difficult to prove beyond all dispute, but the feldspars in the same 
specimen are almost entirely altered to an apparently identical substance, with some 
calcite, while no chlorite or epidote is found, showing that conditions favorable to the 
formation of muscovite certainly existed. In general it can be stated that those 
(specimens in which the development of muscovite is most distinct occurred in masses 
covered by drift or exposed in the workings of mines [86], while those containing 
epidote are from more exposed'positions, usually above timber-line. The observations 
are, however, too few to be considered as indicating any rule in the matter. 

Chemical data A silica determination proves that quartz must be prominent in 
the groundmass, as the quartz macroscopically visible is much less than in the Lincoln 
Porphyry, while the amount of silica found, 05.08 per cent., is but little less than that 
in the latter rock, viz, 66.45 per cent. [85a]. The Sacramento Porphyrite also con- 
tains 3.55 per cent, of soda to 2.57 per cent, of potash [85], which confirms the classi- 
fication as a plagioclase rock. 


Occurrence and previous classification The intrusive sheets of eruptive rock occur- 
ring in Mount Silverheels, two of which appear in the northeastern part of the Mos- 
quito Range map, belong to a rock which is not easily classified. It is colored as 
rhyolite upon the Ilayden Atlas of Colorado, and called ' (trachyte?)" by A. C. Peale 
in his report upon the region. 1 Unfortunately, its relations were not at first correctly 
understood by the present writer, and consequently the rock is colored upon the map as 
quartz-porphyry, while he now regards it as a plagioclastic rock and as belonging to 
the series of porphyrites. 

Description This rock is of a greenish or gray color and very fine grained, but it 

still exhibits a distinct porphyritic structure when not too much decomposed. Its 
macroscopically visible constituents are feldspar, biotite, hornblende, and, sparingly, 
quartz, all of them in very small individuals, seldom exceeding 3""" in diameter. The 
groundmass is usually obscured by chloritic decomposition products. Microscopical 
study shows the usual accessory minerals, including allanite and pyrite. 

With regard to the feldspar crystals it is difficult to decide which may have been 
predominant from simple microscopical study, for many of them are entirely decom- 
posed and the mixture of calcite and muscovite resulting in all cases does not give a 
1 Annual Report United States Geological and Geographical Survey of Territories, 1873, pp. 214-21G. 


certain clew. Au alkali determination in one of the freshest specimens [109] yielded 
Xa 2 O 4.08 per cent, and K 2 O 2.70 per cent., which must be decisive in confirming the pres- 
ent classification, for it is to be expected from analogy with the fresher rocks previously 
described that the larger part of the soda will be contained in the porphyritic crys- 
tals of the first generation, while the potash will remain chiefly in the groundmass. 
When nearly balanced alkalies are, as a matter of fact, so disposed in the solid rock, 
the soda feldspar certainly becomes the more prominent and should determine the 

The groundmass is holocrystalline, in most cases coarsely macrocrystalline, and 
is made up of quartz, orthoclase, and plagioclase, with some biotite. Its constitution 
is often obscured by chlorite. The amount of quartz seems less than in most porphy- 
rites described, and a silica determination gave but 60.42 per cent. [109&]. Epidote 
and chlorite are the products of the decomposition of both biotite and hornblende. 
What seem at first sight to be included fragments of arnphibolite are most probably 
Accretions of hornblende from the magma in an earlier period of the rock's history. 
Biotite and some feldspar accompany the hornblende. 

On the extreme southern spurs of Mount Silverheels, beyond the limits of the 
present map, between the forks of Crooked Creek, a variety of much more distinct 
porphyritic habit was found, which is colored on the Hayden map as "Porphyritic 
Trachyte" [108]. Its crystals reach l om in diameter and predominate over the light- 
grayish groundmass. All the elements are the same in character as in the Silver- 
heels rock, and it can be regarded only as a modification of the same. 


The "Green Porphyry," a peculiar fine-grained rock, was found occurring in three 
different places: first, as a dike, running from the northern edge of Bross amphithea- 
ter toward Mount Silverheels [98a]; secondly, on the north side of Mosquito gulch, 
near its mouth, interbedded in Cambrian quartzites [98]; and thirdly, as an interstrat- 
ified bed on lower Loveland Hill, near the Fanny Barrett and Eagle Bird claims 
[986]. It is macroscopically compact, light gVeen in color, with an abundant chloritic 
decomposition product, which renders it difficult to distinguish clearly each crystal indi- 
vidual, although it is sometimes plain that the rock is almost wholly macrocrystalline. 
Quartz, feldspar, biotite, and hornblende are, however, recognizable, the latter two 
being much altered. 

Some of the thin sections prepared show no normal groundmass at all, although 
a distinction can be made between certain well-crystallized elements and wholly irreg- 
ular fragments. There seem to have been original crystals of feldspar, hornblende, 
and biotite, all quite small, while the remainder of the rock, solidifying later, was 
formed of the same minerals, with quartz, in irregular grains, which sometimes have 
reached the size of the crystals, but more frequently have not. 

The feldspars are largely replaced by muscovite and calcite ; the dark silicates 
by chlorite and epidote. Quartz is not abundant, a silica determination yielding but 
63.85 per cent. A few fluid inclusions are observed in quartz and feldspar. 

In connection with the above rocks should be mentioned several occurrences not 
to be classed under any of the described varieties, though most closely allied to the 


last one [100-106]. In the hand specimen they show but little that can be identified. 
They are green in color and fine-grained, with some visible feldspar and biotite or 
hornblende, and, rarely, quartz. The principal decomposition product is chlorite, which 
renders the structure obscure. The microscope reveals a fully crystalline structure, 
in which a granular groundmass of quartz and feldspar is of varying importance. 
Quartz and orthoclase, intimately but irregularly intergrown, make up in some cases 
the greater part of the rock. Muscovite is the chief decomposition product of the 
feldspars and seems also to result from the alteration of biotite, after several inter- 
mediate stages. 

The Green Porphyry and the ones just mentioned are now thought to be more 
probably porphyrites than quartz- porphyries. 



Among the acid orthoclastic rocks of the district arc a few occurrences plainly 
distinct from any that have been referred to the group of the quartz-porphyries. 
Their mode of occurrence is different (see p. 194) and they possess to an eminent 
degree the habit formerly considered characteristic of the younger eruptives. No exact 
data as to age are available, but they all seem to be more recent than the period of 
folding and faulting. 

The most important body of rhyolite is that upon the northern boundary of the 
region under consideration, forming the mass of Chalk Mountain. As this rock has 
very closely the habit of that subdivision of the rhyolites recently denned as Neva- 
dite by Messrs. Hague and Iddings, 1 that name will be applied in this description. Ac- 
cording to the definition of the writers cited, Nevadite is a rhyolite "characterized by 
an abundance of porphyritic crystals imbedded in a relatively small amount of ground- 
mass," while liparite is a rhyolite "characterized by a small number of porphyritic 
crystals imbedded in a relatively large amount of groundraass." These terms simply 
designate structural extremes in a group which is so large as to need some such treat- 
ment. They occupy about the same position as the terms " granite porphyry " and " fel- 
site porphyry." 


General description This rock is characterized by the appearance of very numer- 
ous dark quartz crystals and clear sanidines, with but very little biotite or ore, imbed- 
ded in a light gray groundmass. On the western and eastern parts of the mountain 
the feldspars are nearly all small and clear, and, as in this modification there is an 
almost total absence of biotite and ore particles, the feldspars are scarcely distin- 
guishable at first glance from the enveloping groundtnass, which has under the lens an 
exceedingly fine-grained, homogeneous texture. All this only serves to bring out the 
more strikingly the abundant dark, smoky quartz crystals, which usually present the 
prism in very distinct development. They are here invariably fissured in all direc- 
tions, and fractured surfaces have an unusually brilliant, vitreous luster. In this 
modification of the rock the quartz crystals seldom reach l cm in diameter, and the 
feldspars, though occasionally more than 2 cm in length, are usually less than l cm in 
greatest diameter. 

'American Journal of Science, III, XXVII, p. 461, 1884. 



On the southern edge of tbe mountain and on the northwestern slope the rock 
has an even more striking development than that just described. Both quartz and 
sanidine, but specially the latter, occur in large crystals, and, while the quartz is dark, 
as before, the sauidine possesses a most beautiful, brilliant, satiny luster upon a sur- 
face nearly parallel to the orthopinacoid, which is particularly marked in fractured 
crystals. At the same time biotite and ore specks appear in sufficient quantity in the 
subordinate groundmass to give it a tinge of gray and cause it to stand out plainly 
from the feldspars. The dark, smoky tinge of the quartzes, the delicate but brilliant 
luster of the sauidiues, together with the general freshness of all constituents, give 
to the rock an extraordinarily beautiful appearance. On Plate VIII, page 88, is a 
heliotype representation of this Nevadite, which but feebly expresses the strong con- 
trast between various constituents. 

Macroscopic constituents Of the feldspars in this rock only the sauidine is at all 
prominent, although plagioclase appears in small crystals and sparingly in the ground- 
mass. The plagioclase must be an oligoclase poor in lime, as is shown by the rock analy- 
sis later. Sauidiue, much more glassy and fresh in appearance than the plagioclase, is 
by far the most interesting component of the rock. Many of its crystals are Carlsbad 
twins, sometimes polysynthetic, and exhibit the faces oc P, co P &, OP, and 2P ob. The 
luster which has been mentioned is highly characteristic and is described in detail 

Some of the large, lustrous sanidiues exhibit a peculiar internal structure. On 
breaking open several crystals there appeared a kernel partially detached from an 
outer zone or shell about l mm in thickness. All free surfaces of the kernel are glis- 
tening crystal faces, and the inner surfaces of the shell are likewise regular crystal 
planes, upon which miuuto projections are found to be like attached crystals, with the 
same orientation as the larger individual. The shell usually exhibits the satiny luster 
more markedly than the kernel, but no other difference was noticed between the 
substance of the two parts. 

From a clear crystal in which the luster was not pronounced a section was pre- 
pared nearly at right angles to the edge between OP and P do, and the optical axes 
were found to lie near together in a plane normal to so P <x. 

The quartz crystals and grains of this rock are quite free from mineral inclu- 
sions; glass has never been observed in them and arms or inclusions of the ground- 
mass are alike nire. Gas pores are, on the other hand, quite abundant, being in part 
negative crystalline in form. Many pores, seeming at a low power to be merely filled 
with gas, are really fluid-inclusions with a relatively small amount of fluid. This is 
very plain if the cavity is irregular, the fluid being pressed into the angles or pro- 
jecting arms, while the main part of the cavity is occupied by the bubble. 

Biotite is very sparingly developed in small hexagonal leaves. Magnetite is the 
only ore mineral and is present in very small quantity. Apatite and zircon are the 
remaining accessories, and both are much less abundant than is usual in the rocks of 
the district. 

The groundmass Quartz and feldspar in a very even-giained mixture are almost 
the sole constituents of the grouudmass. In the coarser variety of the Nevadite the 
average size of the grains is 0.02 mm to 0.05 mm , and the greater part can be identified as 


quartz or feldspar, the larger portion of the latter being inonoclinic. The groundmass 
of the more compact varieties of the rock is cryptocrystalliue. Gas pores of irregu- 
liir shape are present between the granules in all modifications. 

While there is no niicrofelsitic substance and no persistent glassy base, properly 
speaking, there are irregular, disconnected patches or particles of a clear, structure- 
less, isotropic matter, with branching arms filling spaces between grains of the ground- 
mass. This substance is most clearly developed in the coarser parts of the rock mass, 
and it is apparently identical in character with the glass observed in the rhyolite from 
the Hohenburg near Berknm, on the Bhine, Germany, first described by Zirkel. 1 In 
manner of occurrence of this glass residue the two rocks are very similar, though it is 
more abundant in the German rock. The latter contains plagioclase abundantly in 
the groundmass and its basic silicate is hornblende. 

Drusy cavities In the coarser -grained parts of theNevadite body are numerous 
small cavities lined by minute crystals. At the northwestern point of Chalk Mount- 
ain they reach the maximum in size, some observed being several centimeters in great- 
est diameter. In the larger ones the crystals reach a determinable size and are found 
to be chiefly sanidine, in delicate glassy tablets that are always Carlsbad twins, 
with some quartz, biotite, and topaz. A few stout crystals seem likely to be triclinic 
feldspar, but they could not be definitely determined. A coating of manganese binox- 
ide is often upon the crystals and dark spots in ihe mass of the rock seem due to 
the same substance. Both sanidine and topaz from these druses are worthy of special 
notice and are described below. No minerals which can be considered alteration prod- 
ucts are found in these druses and a natural explanation for the occurrence is to 
regard all the crystals as sublimation products. 

Topaz Usually but a single topaz is present in one of the druses, and that is 
larger and more perfect in development than any other crystal. The topaz is attached 
directly to the walls of the cavity and often bears small tablets of sanidine upon it. 
The crystals which can be recognized vary from 0.5 mm to 3 mm in length, but it seems 
quite probable that there are some smaller ones, indistinguishable from quartz. 

The determination rests upon the crystalline form, which is very distinct and is 
that of common topaz. One crystal, measuring S"" 11 in length and l mm in thickness, 
was removed from the rock, and its angles were measured with a Fuess reflection 
goniometer. This crystal presents oo I 3 , o= ?2 and 2Poo as the dominant forms; OP is 
a narrow face and 4? 00 , 2P co , 2P, and P are minute, but very distinct. The angles 
measured are as follows: 

ooP A*P 124 16' 

ooPSA^P aoveroofoo 93 7' 

OP A 2 foe 136 30' 

OP A P 1* 11' 

OP A 2P 115 55' 

2P oo appears as a very narrow face in the zone of 2P to 2P. This is the usual habit, 
with the occasional addition of oo ? oo , and a more prominent development of OP. 
This crystal is also imperfectly terminated at the attached end, showing 2p oo most 
prominently, with 4f' oo and 2P also recognizable, and there are no signs of henii- 

1 Mik. Beschaf. der Mill, uud Gesteiue, p. 343. 


No occurrence of topaz in eruptive rocks has been previously described, so far 
as is known to the writer. Topaz is found in other parts of the Kocky Mountains, 
and iu Mexico, where eruptive rocks are said to occur, but the connection between 
the two has not been demonstrated. 

The satin-like luster of the sanidines. The lustrous surface is in the orthodiagonal 
zone and inclined a few degrees to the orthopinaeoid, as is evident iu the Carlsbad 
twins, usually polysyuthetic, the luster reaching its maximum of brightness simul- 
taneously iu alternate planes. Microscopical investigation shows a most perfect parting 
parallel to the surface of luster, and with a knife-blade flakes can be split oft' in this 
direction, even more readily than parallel to the basal cleavage-plane. Thin plates 
parallel to the base (OP) show a very fine striation at right angles to the Hue of cc P So 
and to the directions of extinction. Thin flakes split ott' parallel to the lustrous 
surface show, under the inicioscope, that the luster is due to interference of light iu 
passing the films of air between the extremely thin plates produced by the parting. 
The thinnest flakes, composed of a few plates, are transparent and exhibit delicate 
colors of interference, while those composed of more plates arc dull translucent or 
opaque, the light having been completely extinguished by the repeated interference. 
The luster is then due to reflected light from the air tiluis near the surface and to its 
interference. By examination with a good hand leus, a delicate play of colors may 
be seen upon the lustrous surface of the crystals. 

In the drusy cavities above described the sanidiues are thin tablets, almost invari- 
ably Carlsbad twins, with prominent development of the clinopinacoid. Such crystals 
examined under the microscope, as they lie upon the predominant 
pinacoidal face, aflbrd a means of determining approximately the 
position of the plane parallel to which the parting referred to takes 
place. The adjoining cut represents one of these crystals, a nor- 
mal Carlsbad twin, with a third and smaller plate, also in twin posi- 
tion. The faces shown are : oo P, P 3o , P oo, OP, and 2P ,as indi- 
cated. From all the outlines and from basal cleavage or irregular 
fissures run dark lines, in uniform direction for each individual of 
the twin, and penetrating varying distances into the crystal. This 
undoubtedly represents an incipient stage of that parting, which, 
' the large crystals of the rock, occasions the brilliant luster, for 
t hrse dark lines do not represent ueedles of any mineral substance, 
but the air films filling the fissures. 

This parting may be seen upon all microscopic sauidine crystals of the rock, and 
even the irregular grains of that mineral in the grouudmass, when cut in the right 
direction, show a very fine, delicate striation, which is undoubtedly due to the same 
cause. As seen from the figure, the position of the surface is that of a positive hemi- 
orthodome, for the cleavage plates of large crystals show the plane to be at right angles 
to the clinopinacoid. Assuming the axial ratio 

a : I : c = 0.653 : 1 : 0.552 and ft = 64, 

as determined by Striiver, 1 for free crystals of sanidine, the face corresponds closely to 

J/-P*. This would require an angle of ^2 40' with the basal plane, while that 

'Cited by Tscbermak, Lehrbucli dcr Mineralogie, p. 455, 1*3. * 


measured iu the crystal figured was 72 53'. Of course this canuot be regarded, under 
the circumstances, as anything more than an approximate determination. 

Chemical composition. The specimen subjected to quantitative analysis is from 
the northeastern part of Chalk Mountain [397] and is of the relatively finer-grained 
modification. This was chosen iu order to obtain more easily an average sample of 
the rock. The analysis is by W. F. Hillebrand. 

SiO 2 74.45 

A1. 2 3 14.72 

KcoO 3 None 

FeO 0.56 

MnO 2 0.28 

CaO 0.83 

MgO 0.37 

K. 2 O 4.53 

Na 2 3.97 

Li 2 O Trace 

H 2 O 0.6C 

P,0. 0.01 

100.38 ' . 

The rarity of biotite and of magnetite in this rock, which has already been 
emphasized, is certainly confirmed by this analysis. In fact, it is evident that no 
minerals, aside from quartz and feldspar, play any important role. The large amount 
of soda shown by the analysis made it desirable to know how large a share of it was 
contained in the sanidine, and an analysis of a large clear crystal was therefore made. 
The result was as follows : 


0. 79 

K 2 O 


Na. 2 O ) 

4 11 

LijO 5 ' 
H 2 O 


100. 37 

From this it may be safely assumed that a large part of the soda found in the rock 
belongs to the sanidiue, for no visible impurities were present, such as plagioclase 
grains. The same holds true for the lime. It is worthy of note that the silica per- 
centage is the highest yet obtained in any rock of the region. 


The rhyolite forming Black Hill, in the southeastern corner of the mapped dis- 
trict, is like the Chalk Mountain Nevadite in being composed almost wholly of quartz 
and feldspar, but the resemblance otherwise is not very marked, for the Black Hill 
rock possesses a groundmass which is fully equal quantitatively to the small imbedded 


crystals. Both feldspars are present in numerous crystals, but orthoclase alone is 
prominent in the groundmass. Quartz occurs in abundant, slightly smoky crystals. 
Biotite, in small hexagonal leaves, is sparingly scattered through the whole, and mag- 
netite is also insignificant as a constituent. 

Fluid inclusions appear iu both orthoclase and quartz, particularly in the latter, 
and sometimes carry white cubes, apparently of salt. There are glass inclusions also 
in the quartz, but not plentifully. The groundmass is granular and shows no glass 
substance like that iu the Nevadite. 

The orthoclase, though fresh looking, has none of the glassy appearance of 
sanidine, and it< must be confessed that there is little evidence in the observed charac- 
teristics of the rock demanding that it be separated from the quartz-porphyries. There 
is no direct evidence of its age, and its classification as a younger rock rests chiefly 
upon the following facts. In mode of occurrence and in composition it is more nearly 
related to the Chalk Mountain Nevadite than, to any other rock of the region described. 
It lies separated by a considerable space from all other eruptives of the map, but is 
adjoined at no great distance on the south and southeast by a large series of rhyolites 
and andesites. It is regarded as most probably related to these in its origin. A silica 
determination in fresh rock gave 69.54 per cent. [140]. 


Occurrence On the northern boundary of the area mapped, at the western base 
of Bartlett Mountain, occurs a rhyolite of peculiar character. It appears in one large 
and several small bodies at the head of McNnlty gulch (not indicated on the map), 
which runs north and enters the Ten-Mile River at Carbonateville. White Ridge, 
between Chalk ranch and Chalk Mountain, is also formed of this rock, as are one or 
two minor bodies west of Chalk Mountain, which are not shown upon the map. 

At the head of McNulty gulch this rock cuts porphyrite and the fresh-looking 
quartz-porphyry which < ccurs in the synclinal fold at this point. All these rocks 
extend northward into the Ten-Mile district, and they will be more fully treated iu the 
forthcoming report upon that region. 

Description in the largest body of this rhyolite, indicated upon the map, the 
prevailing habit is that of a light-colored rock, showing numerous slightly pinkish 
quartz crystals, white glassy feldspars, and bright brown uiotite leaves, with a subor- 
dinate ashen-gray groundmass between them. Few crystals exceed 0.5 cm in diameter, 
and the average is much less. Intimately associated with the above variety, usually 
in alternating bands or streams, with rapid though gradual transitions, is a darker 
modification, in which the development of the quartz in particular has been hindered, 
while feldspar and biotite are abundant in smaller individuals than before. The ground- 
mass becomes at once more prominent and darker brown in color, determining the 
general hue of the rock. The thicker these dark portions are the more completely the 
quartz disappears. In the most compact parts of the rock a fluidal structure is macro- 
scopically visible and small glistening prisms of hornblende appear. About included 
fragments of sandstones, etc., this rhyolite grows compact iu a similar manner, and 
also on the contact with wall rock. 


The smaller masses, though sometimes light-colored, seldom contain much macro- 
scopically visible quartz, and hornblende is usually more or less abundant with the 

Microscopical The quartz- bearing variety shows under the microscope a decided 
preponderance of sanidine over plagioclase. The former is in most cases in fragments 
of crystals, while the plagioclase is often in well-defined individuals. A few glass 
inclusions were seen in both quartz and feldspar, while no fluid inclusions were noticed. 
Apatite and magnetite are rather sparingly present. No hornblende could be found 
accompanying the biotite in this form. The groundmass is cryptocrystalline and is 
made up of colorless grains and ferritic specks which are undeterminable. It seems 
probable that there is some microfelsitic matter present, but it could not be definitely 
recognized and there is certainly no glass base. 

The microscope shows almost as much hornblende as biotite in the compact rock 
and there is also a larger determiuable amount of plagioclase than in the preceding 
variety, with the same distinction noticed before in contrast with the sanidine, viz, 
that the latter mineral is more frequently in a fragmentary state, while the former is 
well crystallized. Quartz is present in clusters of small irregular grains and rarely 
in crystals. The groundmass is, as before, cryptocrystalline, but its component par- 
ticles are often minute prisms or flakes and there are more yellowish or opaque grains. 

In the darkest modifications a small amount of quartz can always be recognized, 
but by no means enough to represent the crystals of the light-colored variety. Still 
everything seems to indicate that the various forms are modifications of one magma 
and do not differ greatly in chemical composition. So far as the silica is concerned, the 
truth of this idea was fully established by three determinations made, respectively, in 
the quartz-bearing variety, the compact form associated directly with it, aud, thirdly, 
in a very light-colored rock from a small isolated occurrence not visibly connected 
with any other. These yielded, in the order named, 65.75 per cent., 65.21 per cent., 
and 65.63 per cent, of silica. 


About opposite the Long and Derry mine, on the south side of Empire gulch, 
there is a small body of rhyolite occurring as a bed below the Silurian Limestone [268]. 
This is unlike any other of the rocks examined and deserves a short description. It 
is white, barring the specks of biotite, and very fine-grained, although the lens shows 
man y clear and sharp quartz crystals. The feldspar is distinguishable from the ground- 
mass through its superior whiteness and is apparently no longer fresh. The average 
size of the visible crystals is about l mm . 

Under the microscope the minute quartz crystals are seen to be well-shaped and 
to contain very characteristic clear glass inclusions, with none of fluid or groundmass, 
and a very few apatite needles. The feldspars are chiefly ortboclase, though accom- 
panied by plagioclase, and both seem to be much altered, calcite being the most 
prominent decomposition product. They contain some inclusions of glass, now much 
devitrified. The biotite is fresh and characteristic. Magnetite is very sparingly 


The groundinass has a mottled appearance in ordinary light through the gathering 
of exceedingly minute brownish particles about certain centers, but no optical proofs 
of a radiate structure could be detected. The quartz crystals are surro unded by a 
zone of similar constitution. Seen in polarized light, the whole groundmas_s seems 
cryptocrystalline, no isotropic matter being visible. The substances forming it could 
not be identified, and they seem to be rather needle-shaped or foliate than granular. 
An alkali determination in this rock gave 3.50 per cent, of potash aud 2.17 per cent, 
of soda, while the silica was determined at 68.05 per cent., thus confirming the iden- 
tification as an acid orthoclastic rock. 


Rhyolitic tufa in South Park Four miles south of Fairplay and one mile east of 
the limit of the map is a small outcrop of rhyolitic tufa occurring in the red sand- 
stones of the Upper Carboniferous [141]. It is of very limited extent and is appar- 
ently the extremity of an arm reaching out from some of the larger masses of rliy- 
olite lying to the south or east. It is of a pink color, very light aud porous, aud 
includes many fragments of sandstone as well as pieces of a still lighter tufa. Glassy 
feldspar, swarming with delicate glass inclusions, quartz, biotite, and hornblende, can 
all be recognized. The cementing matter is dull, stained, fibrous, and largely micro- 
felsite. The tufa contains 70..'i per cent, of silica. 

Dike in the Ten-Mile amphitheater A rock which seems to be related to the Chalk 
Mountain Nevadite occurs in the amphitheater forming the source of Ten-Mile Creek, 
just east of Chalk Mountain [139]. It appears as a dike in the Archeau, for the amphi- 
theater lies immediately east of the great Mosquito fault. On account of decompo- 
sition of the feldspars, forming a light greenish-yellow mica, the exact parallelism be- 
tween the two rocks cannot be absolutely established. The macroscopical appearance 
suggests an intimate relationship. 

Breccia in the Eureka shaft in the Eureka shaft, Stray Horse gulch, near Lead- 
ville, a brecciated material was found, in which, among other rocks, is a rhyolite con- 
taining biotite and larger crystals of feldspar than the type from Empire gulch, but 
with a similar groundmass [204]. The sanidiues abound in glass inclusions, and, 
besides the quartz, which is not specially abundant, there are aggregates of tridymite. 


At the head of Little Union gulch, south of Leadville, a rock was found travers- 
ing the Archean and Lower Quartzite in an irregular dike, which must be regarded as 
a quartz bearing trachyte [142]. Owing to its small ;irea and minor geological signifi- 
cance, it has not been designated by a distinct color on the map, but has been included 
under that of rhyolite. 

Its macroscopical appearance is very different from that of any other rock of the 
region. The color is dark gray, its most prominent constituent being a glistening- 
brown biotite, with small glassy feldspars aud a number of rounded yellowish quartz 
grains. Between these is an ill defined, gray groundmass, which is quantitatively 
much subordinate to the crystalline constituents. None of the crystals exceeds 0.5 cm 
in diameter. 


Microscopical Ortboclase (sauidine) and plagioclase seem nearly equal in impor- 
tance. Both are very fresh and in most cases contain few interpositions, although a 
few crystals carry a very large number of devitrifled inclusions. Hornblende in yel- 
lowish-green individuals is quite plenty beside the biotite and both minerals are 
fresh. The amount of quartz seems limited to the macroscopically visible, rounded 
grains, and these, by their freedom from all inclusions and worn appearance, seem like 
accidental rather than normal constituents of the rock. Their number is small, and 
even if original it seems more proper to consider them as accessory. A silica deter- 
mination of an average specimen yielded but 61.22 per cent., so that it is evidently not 
to be classed with the acidic group. Magnetite is abundant, as well as pale mineral in 
irregular oblong grains, which may be titanite. Apatite is inclosed in all the larger 
elements excepting the quartz. 

The grouudmass is microfelsitic in large degree and contains few crystalline par- 
ticles. It shows a distinct fluidal structure, made plain by the contrast between the 
portions carrying indistinct brown needles and colorless portions. The needles are 
sometimes grouped in an imperfectly radiate manner about some small crystal, in a 
manner similar to that in felso-spherulites. These act feebly on polarized light, giving 
a faint black cross when seen between crossed nicols. Some colorless isotropic spots 
seem to be glass. The movements which produced the fluidal structure are also indi- 
cated by the crumpled biotite leaves and broken hornblende prisms. 


Andesitic rocks have not been found within the limits of the Mosquito Eange 
map, but at the Buffalo Peaks, a few miles south, a large variety occurs. In the course 
of a hurried trip a number of specimens were collected from this locality, represent- 
ing several types; of these, two are sufficiently marked in character and occurrence 
to merit particular notice. 


Macroscopical The rock which in the form of a sheet caps the mountain is a pro- 
nounced hornblende-audesite [143]. Macroscopically it is dark brown in color and 
contains feldspar in clear or ashen-gray crystals and dark, glistening hornblende 
prisms as most prominent constituents. With the aid of the lens green prisms of 
pyroxene and ore particles are quite abundantly visible. The brownish grouudmass, 
which gives tone to the whole rock, is rather more abundant than all the crystals 

Microscopical Under the microscope the hornblende is found to possess the usual 
characteristics of that mineral in such rocks. It has a dark, granular border, or is 
occasionally entirely replaced by a mass containing opaque ore grains, augite prisms, 
and some calcite as secondary elements. Besides the hornblende appear both hypers- 
thene and augite, in smaller crystals, but more numerous. The former of these 
minerals possesses the same characteristics as in the accompanying hypersthene- 
audesite. Most of the feldspars are distinctly plagioclase and some of them contain 
irregular glass-inclusions in great number, many of which are now much devitrified. 
Magnetite and large dusty apatite prisms are sparingly present among the porphy- 
ritfcally imbedded crystals. 



The groundroass is a mixture of delicate plagioclase staves, minute prisms of 
hypersthene and augite, with magnetite anil a scanty glass base between them, the 
latter devitrifled by brownish globulites. 


On the northeast shoulder of the mountain a very dark compact rock occurs, which 
seems to be an almost typical angite-andesite. Macroscopically there are numerous 
small glassy feldspars visible and a few green grains and ore specks, but the black, 
generally vitreous groundmass is much more prominent [144]. The microscopical ex- 
amination shows a very close resemblance to the well-known Hungarian " augite- 
andesites " of similar macroscopical habit. The rock contains no hornblende and 
no biotite, while the pyroxene consists in part of hypersthene and in part of common 
augite. Hypersthene is the more characteristic bisilicate in this rock, and the name is 
therefore given as above. Its determination rests upon careful optical and chemical 

Comparative study in connection with the above rock has shown that a large 
number of so-called augite-andesites, both in this country and in Europe, are more cor- 
rectly to be considered as hypersthene-andesites. Detailed investigations in regard 
to the Buffalo Peak rock and a comparative microscopical examination of allied occur- 
rences are given in Bulletin No. 1 of the series published by the United States Geo- 
logical Survey, ''On Hypersthene- Andesite,'' &c. 

On Plate XXI are heliotype reproductions of photographs showing the com- 
position and structure of the chief andesite types of the Buffalo Peaks. The result 
is so unsatisfactory that the figures convey but an indistinct impression. In Fig. 2, 
however, the small prisms of hypersthene are distinguishable from augite, which occurs 
chiefly in peculiar aggregates, with magnetite, feldspar, and sometimes with biotite, 
as shown in the lower left-hand portion of the figure. 


The tufaceous rocks of the Buffalo Peaks are chiefly if not entirely of ande- 
sitic character, although they exhibit a very wide range in composition and texture. 
Some of them are loose or friable ash-beds,.others contain a large amount of dark 
pearlitic glass with the ashy material, and still other beds are so compact as to resem- 
ble massive rocks. In composition they vary greatly, especially in regard to the more 
basic silicates, for hornblende, biotite, hypersthene, and augite are respectively the 
characteristic minerals in different beds, while they frequently occur t< gether. 

The pebbles included in these tufas represent as many types of massive ande- 
sites as are indicated by the various beds of tufa. Granite is also frequently found, 
especially in some layers, and sometimes in large bowlders. 


In the following lines are brought together, in concise form, the results and par- 
ticular features of the preceding description which are deemed of special importance 
or interest: 



Fig. I 

Fia. 2 

Fig. 3 

Fig- 4 

Selwtype Printing Q>. 2U Trenwnr StSosten 

RESUME. 355 


But three granular rocks were found, all of them diorites. In striking con- 
trast to this rarity, it is observed that all the numerous quartz-porphyries and por- 
phyrites are holocrystalline and that the groundmass is in nearly all cases evenly 
granular. Although these rocks occur both in dikes and in relatively large masses, 
this markedly crystalline structure is wonderfully persistent through the extent of 
the existing variation in conditions. 


Of the various rock types described, the following seem specially noteworthy: 

1. white Porphyry This rock illustrates a transition stage between the granu- 
lar and porphyritic structures. Its imbedded crystals are few and small, but they 
evidently correspond to the more prominent constituents of the typical porphyry, 
whether viewed from the structural standpoint or considered in the light of the genetic 
principle discussed in the introduction. In mineralogical composition this is an inter- 
esting type, because of the absence of biotite or a bisilicate as an essential constitu- 
ent. Even the intimate relationship to a biotite-bearing rock indicates nothing more 
than the possible presence of biotite in very insignificant quantity. The common acces- 
sories, apatite and magnetite, are also very rare. 

2. Lincoln Porphyry This widely distributed type is remarkable for its large 
orthoclase crystals, developed during the later stages of consolidation, in the presence of 
abundant plagioclase. The persistency with which these crystals are found in masses 
of various conditions of occurrence gives at first a somewhat erroneous impression as 
to the distinctness of the type. Only the observance of many occurrences leads to a 
correct understanding of the relations of this rock. 

3. Nevadite This variety has solidified at a stage seldom illustrated by instances 
.which have been previously described. In its granular groundmass, consisting almost 
wholly of quartz and orthoclase, are still a few isolated particles of clear glass, a case 
directly analogous to but one occurrence known to the writer. The present form 
may be considered as a fair type of the division of the rhyolite called "Nevadite" 
by Messrs. Hague and Iddings. The peculiar mineralogical components are referred 
to below, and a glance at the quantitative analysis will show a wonderfully simple 
chemical constitution. Silica, alumina, potash, and soda make up 97.67 per cent, 
of the whole, no other element reaching 1 per cent. 

4. Hypersthene-bearing andesite The rocks from the Buffalo Peaks, in which hyper- 
sthene 1 was identified as a prominent constituent, are especially noteworthy only 
as the first ones in America in which the important role played by that mineral was 
recognized. The experience of the last two years has shown the writer that andesites 
containing hyperstheue as an essential constituent are very abundant in Southwestern 
Colorado, while their distribution in the Great Basin and among the volcanoes of the 
Pacific coast has been shown by the publications of Messrs. Hague and Iddings 2 and 
Diller. 3 

1 Bulletin No. 1, United States Geological Survey, 1883. 

'American Journal of Science, III, XXVI, 222, Ih83. Idem., XXVII, 453, 1884. 

3 Americau Journal of Science, III, XXVIII, 252, 1884. 



The large number of porphyrites constitute a series connecting the most dis- 
tinctly plagioclastic forms with those in which orthoclase assumes a very prominent 
place by virtue of its abundant large crystals. The full significance of this transi- 
tion will be shown in a forthcoming report upon the Ten-Mile mining district, which 
lies immediately north of the Leadville region. 


During the study of the eruptives which have been described several constitu- 
ents were found to possess unusual development, while some of great rarity were 

1. Lustrous sanidine The sanidine of the Chalk Mountain Nevadite is charac- 
terized by a delicate but perfect parting, parallel to a plane in the orthozone, deter- 
mined approximately as *f- P^ . When this parting is highly developed it causes a 
brilliant satiuy luster parallel to the plane of parting. (See p. 348.) 

2. Zircon Minute but highly perfect crystals of nearly colorless zircon are regu- 
larly, and sometimes abundantly, scattered through nearly all of the rocks described. 

3. Allanite The main group of the quartz-porphyries and porphyrites contains 
allanite regularly, but sparsely, distributed through it. With the exception of the 
contemporaneous identification by Mr. Idclings, no instance of the occurrence of this 
mineral in such rocks is known to the writer. (See p. 329.) 

4. Topaz This does not appear as a rock constituent proper, but is found in 
drusy cavities in the Nevadite. It is associated here with quartz, sanidine, and biotite 
crystals and seems to be a sublimation product. (See p. 347.) 

5. Orthoclase fragments in a dark porphyrite containing abundant hornblende 
and biotite and occurring as a dike in the Archean, were found numerous pebble-like 
fragments of orthoclase, each belonging to a single individual and unlike anything 
observed in other rocks of the region. These rounded pieces are analogous to worn 
fragments of foreign rocks often seen in neighboring dikes, but their true nature could 
not be definitely established. (See p. 339). 


Notwithstanding the uniform and simple composition of the rocks described, a 
few points of great interest were observed in connection with the decomposition of 
their constituents. 

i. The Sacramento Porphyrite illustrates the tendency to the formation of a siugle 
end product from all the chief decomposable elements, to a degree hitherto unknown 
to the writer, either in literature or in personal experience. Some specimens of this 
rock show epidote replacing hornblende, biotite, orthoclase, and plagioclase, all other 
secondary products being comparatively insignificant in these cases. In certain other 
-.specimens of the same rock a common result of the decomposition of biotite, ortho- 
clase, and plagioclase is muscovite, epidote and all other alteration products being 
here subordinate. (See p. 341.) 

RESUME. 357 

2. Muscovite from biotite The unusual process by which biotite is replaced by a 
mineral indistinguishable from the adjacent decomposition product of orthoclase is 
further illustrated in the Mount Zion (p. 324), Lincoln (p. 330), and Mosquito (p. 328) 
Porphyries. The intermediate stages are referred to in the text. 

3. Epidote. This mineral undoubtedly replaces both feldspars in several rocks 
where no intermediate stage can be seen. While the chemical replacement of ortho- 
clase substance by epidote is not easily understood, it is a fact that the replacement 
does occur when the conditions, whatever they may be, are favorable (p. 341). 

4. Hornblende outlines The Gray Porphyry has fresh or partially decomposed 
biotite, while containing evidences that hornblende was a former constituent, although 
it is now always represented by various extreme decomposition products in areas hav- 
ing the characteristic outline of hornblende. This hornblende must represent an early 
product of consolidation, destroyed in the manner commonly noticed in andesites, and 
both its former existence and its destruction are very probably connected with the 
fact that the Gray Porphyry sheet at Leadville is 12 miles away from the eruptive 
channel upon Eagle River. (See p. 331.) 

5. Rutile and anatase from biotite The early stages of the decomposition of biotite 
are usually accompanied by the formation of yellow needles or of small, apparently 
tetragonal tablets, or of both forms. The identity of the former with rutile is ex- 
ceedingly probable, as they are often twinned in the characteristic manner and cor- 
respond to what have been elsewhere identified. The nature of the latter forms is 
less easily shown, but they agree well with the descriptions of anatase by Diller, * 
while the association with the needles seems confirmatory of this determination. 


The absence of certain minerals as constituents in some cases is worthy of note. 
Thus in the White Porphyry no biotite or bisilicate appears, even in small quantity j 
apatite is very rare in the same rock and seems to be wanting entirely in the Pyritif- 
erous Porphyry; angite appears in a single rock, and olivine-bearing types are wholly 


The simple composition of the Nevadite and of the White Porphyry has been 
referred to above. The relations of the types are noteworthy and will be apparent 
from an examination of the accompanying table, in which the analyses previously 
given are reproduced. 

1 Diller, J. 8. Neues Jahrbuch fur Miueralogie, etc., I, 187, 1883. 



Analyses of eruptite rocks. 

SiOi TiO 

AliOi FeiOj 

FeO MnO 












Mount Zion Por- 
phyry, p. 323.. 

White Porphy- 
ry, p. 324 

Mosquito Por- 
phyry, p. 327 . . 

(W 01 














a 50 










Lincoln Porphy- 
ry p. 328 



















Gray Porphyry, 
p 330 

Sacramento Por- 
phyrite, p. 341 

SilverheeU Por- 
phyrito, p. 342. 

Hornblende mica 
p. 340 

Biotite Porphy-> 
rite, p. 340... J 

Novadito, p. 345. 

Rh.ToHte. Em- 
pire <; ni rii. 
p. 351 












k 83 















The Henry Mountain rocks are of two principal classes, one horublendic, the 
other angitic, with plagioclase as the predominant feldspar in both cases. 


Macroscopical The horublendic varieties have as a class a much more recent ap- 
pearance than theMosquitoKangeporphyrites r with which they agree in composition and 
microscopical structure. This arises from theprevailing light-grayish tone of the ground- 
mass and the glassy luster of the feldspars. Nearly all specimens show a decidedly 
porphyritic structure, although they vary greatly in the relative proportions of the 
groundmass to macroscopic elements. A white or glassy, colorless feldspar, in short, 
stout crystals, or less frequently in tablets, and a glistening dark hornblende are the 
only macroscopic minerals of prominence. A few rounded quartz grains are visible 
in some of the specimens, and pale-yellow, brilliant crystals of titauite can be detected 
in most of them; also, minute ore grains. The gronndmass in which these minerals 
lie is gray or tinged with red when fresh, but is greenish or dull gray when attacked. 
Careful search with the lens shows the characteristic striatiou of triclinic feldspars on 
many individuals, but it is much less prominent than usual. The hornblende is sub- 
ordinate both in size and number of its crystals, and seldom appears in the ground- 
mass in sufficient quantity to give it a greenish tinge, as was common in the Mosquito 
Range porphyrites. Few feldspars reach a diameter greater than l cm , while the 
average is below 0.5 cm . 

Microscopical No. 04 will be first described, as it corresponds so nearly to our 
Mosquito gulch type, and the mutual relations of the two rock groups can thus be made 
most easily apparent. The only minerals appearing in large crystals are feldspar and 
hornblende. No quartz grains fall in the section. Other minerals to be distinguished 
from those in the groundmass are zircon, apatite, magnetite, and possibly some titanic 
iron. An unknown pale-green mineral, polarizing strongly, is present in irregular 
grains in the groundmass (pyroxene?). It is not abundant. 

The feldspar is clearly plagioclase in nearly all cases when seen in polarized 
light. Most crystals show distinct laminse running fully across them, but others con- 

1 These notes were prepared at the request of Mr. Emmons for purposes of comparison with the 
eruptive rocks of the Mosquito Range. The examinations were made upon small specimens and thick 
sections, comparatively few new sections having been made. As the material was in a measure incom- 
plete and is no longer at hand for further study, the notes are presented without elaboration iu sub- 
stantially their original form. The references are to the notes of Capt. C. E. Dutton in G. K. Gilbert's 
report upon the Henry Mountains. 



sist chiefly of one individual, in which a few thin wedges are inserted at one end, or 
on one side, in twinning position. These are, I presume, the crystals described by 
Captain Dutton as orthoclase at one end and plagioclase at the other. A zonal struct- 
ure is often present, which is at times interrupted by the tAvinning. Inclusions in 
the plagioclase are not very abundant. There are sometimes minute dark inclusions, 
regular or irregular in shape and arrangement, which seem to be early inclusions of 
the gronndmass or devitrified matter. Distinct glass and fluid inclusions were seldom 
found. Hornblende occasionally penetrates the feldspar, but inclusions of other min- 
erals are rare. 

The hornblende itself is well developed crystallographically. In this particular 
case (04) it shows an unusual tendency to an alteration, by which dark ore grains are 
formed on and adjoining the outer surface and on cleavage and other fissure planes. 
The appearance is, however, entirely different from that of audesitic hornblende. In 
one place hornblende is apparently forming from pale pyroxene. This is, however, 
an isolated case, as elsewhere the distinct outlines of the hornblende crystals prove 
I hem to be original as such. The hornblende is green and fibrous rather than com- 
pact and yellow. 

Titanite, which appears in most of these rocks, does not seem to be present here 
in good crystals. Apatite is not abundant, but occurs in short, stout prisms. But little 
magnetite occurs in large grains. 

The groundmass is granular throughout and has the same composition as in the 
Mosquito Range porphyrites; that is, it consists chiefly of quartz and orthoclase. 

Of the other Henry Mountain rocks, Nos. 8, 9, 16, 18, 20, 23, 32, 35, 40, 44, 46, 47, 
and 50 thirteen in all seem identical in all essential points with that described 
above. Other accessory minerals appear iu some of these sections. Biotite appears 
as a subordinate constituent in No. 35, corresponding in this respect to our porphyrites, 
and being the sole case noticed. 

Isolated grains of pink garnet occur in Nos. 23, 37, and 47. litanite is present 
in nearly all and ilmenite in some of them. In 40 the latter seems to be producing 
titanite through its alteration. In 46 and 23 (new section) I find allanite correspond- 
ing exactly in appearance to that of the Mosquito rocks. 

It can hardly be asserted that plagioclase predominates in all of the rocks, from 
the evidence of these sections alone, as some of them are very small ; there can be no 
doubt, however, that all belong to the same rock type. I cannot convince myself that 
orthoclase exists in more than isolated crystals among the macroscopic elements. 

Inclusions in feldspar are seldom more numerous or distinct than in the first case 
described. Occasionally, however, a crystal is filled with minute hornblende micro- 
lites and clear crystals of zircon, with other ill-defined matter. 

The feldspars are usually quite fresh, but the hornblende is sometimes entirely 
decomposed. The common result is a mixture of chlorite, filmy calcite, and opaque 
particles. Epidote is often a further product. Granular calcite is visible in some 
cases and its origin doubtful. The minute ore grains of the groundmass are often 
hydrated, giving a dingy tinge to the rock, 

In none of the above rocks can there be any question as to the thoroughly crys- 
talline nature of the groundmass, but it varies in relation to the crystals and in com- 


position. Plagioclase in thin plates may be seen to enter into its constitution and the 
quantity of quartz doubtless varies. It even seems probable th at in extreme cases the 
groundmass may be entirely feldspathic. 

In nine other rocks, 24, 31, 33, 56, 61, 62, 67, 68, and 69, the groundmass is ex- 
tremely fine grained and acts but feebly on polarized light. The granular structure is 
preserved, and I can find no proof of the glassy or strictly microfelsitic base. The 
varying relative quantities of gronndmass and crystals are particularly marked in these 
fine-grained rocks (see 31 and 33). 


The rocks included here are Nos. 28, 43, and one of those numbered 31. Hand 
specimens of 31 and 43 were among those sent. 

Macroscopical Specimen 31 is distinctly porphyritic, the greater part is dull 
ashen-gray in color, and in this portion feldspar and groundmass are not clearly dis- 
tinguishable throughout. There are a few fresh pink feldspars in tabular crystals, 
presumably orthoclase, reaching in one case nearly 2 cm in length. Similar feldspars 
were not noticed in any of the hornblendic rocks. 

The dark basic mineral is very black and occurs in short stout crystals, mostly 
small, which lack the luster of hornblende. A careful examination with the lens 
shows also that the section of the prism is octagonal, with alternate sides but slightly 
developed. This mineral is not so abundant as the hornblende in preceding rocks. 
Glistening ore particles and yellow titanite are distinct, though small. 

Microscopical (Of No. 31.) It is rather difficult to determine the nature of the 
dominant feldspar in this rock. I think it is plagioclase, but cannot say that I can 
prove it from the microscopical examination alone. In the first place, this feldspar does 
not seem to polarize light so strongly as is common. Captain Button probably referred 
to this rock when he said that certain feldspars "had almost ceased to polarize." In 
the second place, those crystals determinable as plagioclase are apparently oligoclase 
of medium composition, for the direction of total extinction in the sections examined 
does not vary far from the line of the twinning plane. It is therefore often difficult to 
recognize the polysynthetic structure. By the aid of the quartz plate many are found 
to be distinctly triclinic, but still so many remain undeterminable that it is possible 
that orthoclase predominates in the rock as a whole. The feldspars resemble those in 
granitic rocks in their dirty appearance, the result of incipient decomposition pro- 
ceeding from innumerable cleavage planes. 

Inclusions of augite are rare. Glass inclusions were not noticed and fluid ones are 
indistinct and rare. The augite is unique in its optical behavior in that it appears as 
bright green by ordinary light and has a pleochroism as strong as is usually found in 
green hornblende, giving, too, almost exactly the same colors. In all other and more 
important respects this mineral shows the characteristics of augite. Contours of 
prism, cleavage, and maximal angle of extinction in prismatic zone (nearly 45) all 
indicate augite. Titanite and magnetite often penetrate the augite. 

The groundmass seems wholly crystalline, yet is unlike that common in the 
hornblendic rocks. It seems composed of feldspar and augite, with no visible quartz. 
The feldspar is chiefly present in tabular particles, and not in irregular grains. The 
pale-green microlites and grains, which are quite abundant, seem to be of augite, as 


there is more or less of a gradation in size from the large ones to these in the ground- 
mass. Very minute ore particles are present. 

No. 43 is of quite different macroscopical structure. It appears almost macrocrys- 
talline, the grouudmass occupying simply the interstices between the small white 
tablets of feldspar, while the augite occurs in minute grains not recognizable by the 
naked eye. 

Microscopical The feldspars have a duller appearance even than those in 31, and 
there is the same difficulty in determining which species predominates. The augite is 
the same in character, but does not appear in the groundmass as in 31. 

In No. 28 (the slide alone examined) exists still another form of structure. The 
whole mass is here microcrystalliue and consists chiefly of feldspar, concerning which 
the same doubts exist as before. The augite is very distinct. The grouudmass is made 
up of small feldspars and nearly every one is determinable as feldspar. Quartz does 
not appear; the same accessory minerals are here as in others, titauitc, magnetite, &c. 
Hornblende is exceedingly rare, if, indeed, it occurs at all in these three rocks. No. 
29, however, shows both minerals. The hand specimen shows large, distinct horn- 
blendes, but in the slide, among the few minute irregular grains (no large ones being 
present), augite appears fully as abundantly as hornblende. The remainder of the 
rock is entirely feldspathic, both orthoclase and plagioclase being recognizable. 

But one rock remains, No. 57. This is the sanidine-trachyte of Dutton. Not hav- 
ing the hand specimen and with only one slide, but little can be made out of it. It 
seems like a tufa or fragiuental rock of some kind. The minerals recognizable (plagio- 
clase, orthoclase, quartz, and hornblende) are chiefly in irregular fragments of crystals 
and the gronndmass, though cryptocrystalliue for the most part, has some isotropic 


. The greater part by far of the Henry Mountain rocks correspond very closely in 
composition and structure to our Mosquito Eauge porphyrites, or in particular to those 
varieties in which biotite is rare or is wanting and in which the hornblende does not 
appear in the grouudmass in large quantity. Both consist of plagioclase and horn- 
blende, with a granular grouudmass, composed essentially of quartz and orthoclase. 
They differ 

a, in outward appearance. 

6, in almost total lack of biotite. 

c, in frequent presence of titanite. 

d, in that the grain of the groundmass sinks in certain cases to exceeding 


None of these is weighty in comparison with the resemblances. 
The outward difference seems due to the fact that the specimens were taken from 
the surface in a region essentially dry and arid. 









CHAPTER I. Ore deposits 3G7 

Classification of ore deposits iu general 367 

Leadville deposits 375 

II. Iron Hill group of mines 3SO 

Irou Hill 380 

North Iron Hill 401 

III. Carbonate Hill group of mines 409 

General structure 409 

Southern group of mines 414 

Northern group of mines 429 

IV. Fryer Hill group of mines 445 

General description 445 

Mine workings 455 

R6smu6 489 

V. Other groups of mines 493 

Mines and prospects in the Leadville region 493 

Mines and prospects outside the Leadville district 519 

VI. Genesis of Leadville deposits 539 

Manner of occurrence 540 

Composition of ores 543 

Composition of vein materials 556 

Ores deposited as sulphides 562 

Mode of formation 565 

Origin or source of the metallic minerals 569 


Tables of analyses and notes on methods employed 589 

Eruptive rocks 589 

Limestones 596 

Ores and vein materials 599 


Argentiferous lead smelting at Leadville 613 

Introduction 613 

Preliminary conditions of smelting 614 

Materials used in smelting 636 

Plant and smelting operations 659 

Products of smelting 692 

Theoretical discussion 731 

Metallurgical plates.. 749 

Central index . 753 




PLATE XXII. Vein phenomena, showing replacement action 420 

FIG. 1. Evening Star Incline. 

2. Glass-Pendery mine. 

3. Carbonate Incline. 

4. Forsaken Incline. 

XXIII. Circular furnace, smelter A r 749 

XXIV. Reverberatory furnace and dust chamber, smelter A .. 749 

XXV. Flue arrangement, smelter A 749 

XXVI. Rectangular furnace, smelter B 749 

XXVII. Circular furnace, smelter B 749 

XXVIII. Bartlett smoke filter McAllister charcoal kiln 749 

XXIX. Rectangular furnace, smelter C 749 

XXX. Dust chamber, smelter C 749 

XXXI. Blast arrangement and elevation of works, smelter C 749 

XXXII. Furnace and dust chamber, smelter D 749 

XXXIII. Furnace and dust chambers, smelters F and M 749 

XXXIV. Dust chamber, smelter F 749 

XXXV. Square furnaces, smelter G 749 

XXXVI. Zones of temperature Elevation of smelter G 749 

XXXVII. Circular furnace, smelter H 749 

XXXVIII. Dust chamber, smelter H Jolly's spring-balance 749 

XXXIX. Assay furnace, smelter H 749 

XL. Dust chamber, smelter J 749 

XLI. Blake crushers 749 

XLII. Blowers Baker's and Root's 749 

XLIII. Assay implements 749 

XLIV. Smelter's implements "49 

XLV. Alileu crusher Furnace aud dnst chamber, smelter I 749 

FIG. 2. Highland Chief mine 501 

3. Colorado Prince and Miner Boy mines 504 

4. Florence mine, Printer Boy Hill 510 

5. Taylor Hill r >ll 

6. El Capitan mine 5 ; H 




The preceding chapters have been devoted almost exclusively to the 
consideration of the geological structure of the district. This subject has 
been treated at considerable length, not only because it presents many facts 
which seemed of sufficient interest to geologists in general to justify such 
treatment, but also because a thorough knowledge of the geological struct- 
ure of a region is an essential and indispensable basis for the study of its 
ore deposits ; a fact which is too often lost sight of by those practically 
engaged in mining. For a time the miner may develop his mine success- 
fully by simply following the ore lead, guided by the empirical rules which 
experience has taught him, and without regard to the geological phenomena 
presented by the country rocks, their structural conditions, or the probable 
origin and manner of formation of the deposits ; but the time is sure to 
come when without this knowledge he will be liable to make mistakes 
which may cost him more than he has gained by all his previous labors. 

Before proceeding to a detailed description of the various ore deposits 
of the region studied in the course of this investigation, it may aid the 
reader to have a brief resume of their principal characteristics and a con- 
cise statement of the conclusions which have been arrived at with regard 
to their origin and manner of formation. 


To a scientific description of natural objects the most valuable aid 
is a rational and universally accepted system of classification. The first 
obstacle one encounters in attempting the description of ore deposits is 



the absence of such a classification. The object of a system of classifica- 
tion is not only to afford a means of avoiding long and repeated circum- 
locutions in descriptions, but also to furnish a comprehensive view of the 
mutual relations of the classes of phenomena to which it is applied. Such 
systems must necessarily change from time to time as the scientific studies 
of the phenomena progress and knowledge with regard to them becomes 
more accurate and thorough. The unsatisfactory state of existing classifi- 
cations of ore deposits is due in large degree to an imperfect knowledge of 
the subject on the part of those who have made them, but in part also to 
their being made from a false standpoint. 

As the study of geology sprang originally from the empirical observa- 
tions of those engaged in mining for the useful metals, so the first systems 
of classification of ore deposits were based on distinctions and character- 
istics established by the miners themselves in their daily work, and, as in 
carrying on this work the outward form of the deposit was the most essen- 
tial characteristic, this naturally formed the basis of their classifications. 
But while general geology has made relatively more rapid progress than the 
study of ore deposits, which, being a matter of practical and economic im- 
portance, has seemed to many to belong to a lower sphere of scientific inves- 
tigation than purely theoretical questions, the prevalent classifications still 
hold largely to the original basis of the practical miner. The form of a 
deposit might well constitute the basis of a classification, if it constituted 
an essential characteristic thereof, and if there were certain regular forms 
that belonged exclusively to particular classes of deposits, which had a 
necessary connection with the sum of their other characteristics. This is 
so far from being the case, however, that not only is no one form confined 
to any particular class of deposit, but the same class of deposit, that is, 
one which has undoubtedly the same origin and manner of formation, may 
have a great variety of different forms, as is the case with those about to 
be described. 

That the scientific study of ore deposits has not kept pace with the 
advance in other branches of geology is due in great part no doubt to the 
inherent difficulty of the subject, but also in a measure to a want of scien- 


tific zeal or knowledge on the part of those who are practically engaged in 
mining. The phenomena to be investigated must be studied in the under- 
ground workings of mines, in which not only is a very small area open to 
observation as compared with the surface phenomena on which other geo- 
logical reasonings are mainly based, but they are not in their nature as 
permanent as are the latter and soon become obscured by decay or entirely 
inaccessible. But, while the attainable facts are thus relatively meager, 
they have not all been made available to the student, for the reason that 
those practically engaged in mining are too often content with noting those 
alone which have an immediate practical bearing, and have neglected to 
put on record those of merely theoretical interest, which, nevertheless, if 
carefully observed, might afford a basis for scientific generalizations of great 
economic importance. 

We can only hope to arrive at a satisfactory and rational classification, 
which shall be founded essentially on genetic principles, when our knowl- 
edge of ore deposits shall be vastly increased by the accumulation of a 
great number of scientific observations, based on correct geological studies, 
and towards this accumulation we must look to those practically conducting 
mines for a most essential contribution, since they alone have the opportu- 
nity of daily observation of the constantly changing phenomena which ore 
deposits present. Meanwhile it may be of use to review some of the more 
prominent systems of classification proposed by modern writers upon ore 
deposits, and to consider their relative applicability to the important class 
of deposits under consideration. 

As the Germans were the first to write upon mines and ore deposits 
and the classifications adopted by other nations have been to a greater or less 
degree founded upon their wt)rk, the first place will be given to a mention 
of those most current in Germany at the present day. The original edition 
of B. von Cotta's treatise upon ore deposits appeared in 1853, and has not 
been essentially changed in the later edition here quoted. The next classi- 
fication quoted is that of Dr. Joh. Grimm, professor of the School of Mines 
in Pfibram, Bohemia. The third is that given in his course on mining at 
the School of Mines of Berlin, by Professor H. Lottner, and published by his 




successor, Professor A. Serlo. The last, that of Dr. A. von Groddeck, of the 
School of Mines at Clausthal, in the Hartz. 

Von Cotta. 1 

Grimm. 2 


Von Groddeck.* 

I. Deposit! nf regular form. 

I. Ditseminatiom or impreg- 

I. Inclosed or underground 

I. Original or primary lie- 

1. Beds. 

nations (deposits form- 



a. Beds of ore, coal, etc. 

ing an essent ial const it 

1. Sheet (regular-shaped) de- 

A. Contemporaneous with 

6. Placer deposits. 

nent of the country 


country rock. 

2. Veins. 


a. Veins. 

1. Deposits in stratified 

a. Transverse or ordinary 

1. Original impregnation!. 

6. Beds. 



2. Secondary impregnations. 

2. Mass (irregular-shaped) 

2. Deposits in eruptive 

b. Bedded veins. 

II. Dittinct ore depolitt 



c. Contact veins. 

(forming an accessory 

a. Stocks. 

B. Later than country rock. 

d. Lenticular veins. 

constituent of the coun- 

b. Stock works. 

3. Deposits filling pre-exist- 

II. Drpoiitl of irregular 

try rock). 

3. Other irregularly-shaped ing Oavitie9 ; " 

form - 1. S:.eet (regular-shaped) 

deposit! (pockets, kid- Vejn8 or ^^ 

1. Stockt (sharply defined maun. 

njs. *'> 6. Cave fillings. 

bodies.) a. Bedded (sedimentary) 

II. Superficial deposits. 4 Metamorphic (or metaso- 

a. Stock works. 
6. Contnct stocks. 
c. Cave fillings, 
d. ande. Pockets, kidney- 

b. Veins; crevice deposits: 
stringers (filling open 

4. Deposits of debris (plac- 

5. Surface deposits in place 

matic) deposits. 
II. Secondarn or detrital de- 

shaped deposits (Bntz- 

c. Sheet-shaped segrega- 

(bog-ore, &c.). 

en, Kachelen, Taschen, 


Neuter, Dinner, Nie- 

2. Stock! and irregularly 


shaped deposit!. 

2. Impregnation! (bodies 

a. Bedded (sedimentary) 

1 Die Lehre von deu ErzlaEerstStten. Freiberg, 1859. 

not sharply defined). 


*Die Lagerstatteu der nutzbaren Mineralien. Prag, 

. Independent impregna- 

ft. Stocks (Butzcn.Nestet, 



etc.), filling pre-exist- 

Bergbaukumle. Berlin, 1878. 

b. Dependent impregna- 

ing cavities. 

Die Lehre von den Lagerst&tten der Erze. Leipzig, 

tions (connected with 

c. Stock works (reticnlat- 


other deposits). 

ed veins). 

Untranslatable miner's 



Von Cotta's- classification is founded exclusively on the form of the 
deposit and recognizes no genetic principle as a basis of classification. Thus, 
such essentially opposed deposits as coal beds and placer deposits, on the 
one hand, and mineral veins and contact deposits, on the other, are put 
\inder one general heading ; while cave-fillings, pockets, etc., which may be 
merely offshoots from a vein or contact deposit, come under a distinct main 

Grimm's classification is also mainly founded on the outward form of 
the deposit, but he admits a few minor genetical distinctions, such as sep- 


arating bedded deposits of sedimentary origin from those which were formed 
later than the inclosing rocks. Lottner also bases his classification on out- 
ward form alone, but distinguishes secondary from original deposits. Von 
Groddeck lays much more stress on genetic distinctions, and not only 
brings in each of those recognized by the two previously named, but admits 
the existence of ore deposits of later formation than the country rock which 
do not necessarily fill pre-existing cavities or fissures. 

F. Posepny, 1 professor at Pfibram, who has made an extensive study 
of ore deposits, including many of those of the United States, proposes an 
even more radically genetic subdivision of metalliferous deposits into (1) 
deposits in pre-existing cavities and (2) those formed by gradual replace- 
ment of the rock substances by the vein material or mineral, the first class 
being further subdivided into those filling cavities formed in a mechanical 
way, or dislocation spaces, and those formed by corrosive action in soluble 
rock, or corrosive spaces, which would correspond in general, though not 
necessarily in all cases, to the distinctions of Grimm and Groddeck of the 
fissure-fillings and cave-fillings. 

In order that a classification should find general acceptation among 
mining men, it is essential, moreover, that it should be simple, concise, and 
of easy comprehension, qualifications which the first two of the above 
systems certainly do not possess. Thus, in this country, where mining 
geology has found its principal discussion in courts of law, in which Prime's 
translation of von Cotta has been generally accepted as authority, ore 
deposits of primary origin (leaving placers out of consideration) are practi- 
cally divided into true fissure veins and deposits which 'are not true fissure 
veins, the latter class being somewhat loosely subdivided into contact 
deposits, blanket deposits, and rake, pipe, and gash veins. 

The term "blanket deposit" is probably derived from the mania of the 
Spanish miners, a term which in Mexico and South America designates the 
richest and most productive ore bodies, but in the United States is apt to 
be applied in rather a derogatory sense to any horizontal sheet of ore. The 
last terms are derived from local usage in the lead regions of the north of 

1 Archiv fur praktische Geologic, p. 600. Wien, 1880. 


England, and their general application is of very doubtful advisability, 
since authorities differ as to their exact definition. The term " gash vein " 
is the only one recognized in the classifications given below, and is there 
applied to a fissure which is confined to a particular rock or bed and which 
does not extend into the adjoining rocks. 

The English literature of ore deposits is even more meager than the 
German. Of general treatises on this subject, the more prominent in this 
country are J. D. Whitney's Metallic Wealth of the United States, pub- 
lished in 1854; an article by R. W. Raymond, in his Mining Statistics for 
1869; and an admirable but little known paper on ore deposits, in John- 
son's Cyclopaedia, by R. Pumpelly. J. S. Newberry has also published an 
article on the origin and classification of ore deposits in the School of Mines 
Quarterly for March, 1880. In England, J. Arthur Phillips published in 
1884 an extended treatise on ore deposits. Of the classifications proposed 
by .the above authors, those of Newberry and Phillips are nearly identical 
with that of Whitney and Raymond's is avowedly an adaptation of Lottner, 
the differences in either case being unessential for the purposes of the pres- 
ent discussion. Those of Whitney and Pumpelly alone are therefore given 
here, and to them is added that given by A. Geikie in his Text Book on 
Geology (London, 1882), mainly because of the different standpoint from 
which it is made. 1 

'Prof. Joseph Le Conte has also published an article on the Genesis of Ore Deposits, in the Amer- 
ican Journal of Science fer July, 1883, in which a subdivision into (1) fissure veins, (2) incipient fissures, 
(3) brecciated veins, (4) substitution veins, (5) contact veius, (6) irregular ore deposits, is given. 



J. D. Whitney. 

K. Pnmpelly. 

A. Geikie. 

I. Superficial. 

I. Surface deposits. 

I. Contemporaneous ores of 



1. Residuary deposits. 

stratified rocks. 

a. Constituting tbe mass 

2. Stream deposits. 

n. Contemporaneous ores of 

of a bed or stratified 

3. Lake and bog deposits. 

crystalline rocks. 


III. Subsequentlyiatroduced 

&. Disseminated through 

II. Forms due to the texture 



sedimentary beds. 
Originally deposited 
from aqueous solution, 
but since metamor- 

of the inclosing rock or 
to its mineral constitu- 
tion, or to both. 

1. Disseminated concentra- 

1. Mineral veius or lodes. 

2. Stocks and stock works 
(including gasb veina). 



III. Tfnstratified. 

a. Impregnations. 

i. Fahlbands. 

a. Masses of eruptive 


2. Aggregatedconcentrations. 


&. Disseminated in erup- 
tive rocks. 

a. Lenticular aggregations. 
b. Irregular masses (stocks) 


c. Stock work deposits. 

e. Reticulated veins (stock 


d. Contact deposits. 


. Fahlbands. 

d. Contact deposits. 


!f. Segregated veins. 

III. Forms due chiefly to pre- 


g. Gash veins. 

existing cavities or open 


h. True or fissure veins. 


1. Cave deposits. 

2. Gash veins. 

3. Fissnre veins. 

All of the above are an advance upon von Gotta in that form is not in all 
cases the exclusive basis of classification. Whitney's first two subdivisions 
are distinctly genetic, but the third, which embraces the majority of metal- 
liferous deposits, is an unsystematic grouping of a variety of forms having 
only one' common quality, that of not being stratified. Whitney recog- 
nizes a genetic quality in his division , that of being of eruptive origin, but 
few geologists of the present day agree with his wide application of this 
quality for instance, to the great deposits of magnetic iron of Missouri and 
Lake Superior. In his " segregated veins " he recognizes the possibility 
of an unstratified deposit which is not the filling of a pre-existing cavity, 
while no such recognition is found in Geikie's classification. Geikie's term 
"subsequently introduced ores," on the other hand, is to be preferred to 
" unstratified deposits," as being based on a more essential characteristic 
of the deposit. This would involve, however, a definite statement as to 
the age of Whitney's Classes III, a and 6, which his general term avoids. 


Pumpelly's classification ignores the division of stratified or contem- 
poraneous ore deposits, and in his text he states his belief that the greater 
number of ore deposits have been formed later than the inclosing rock ; 
he also says that all metalliferous aggregations are the result of a proces.8 
or series of processes of concentration. 

Posepny states his opinion on contemporaneous deposits even more 
strongly in the following words : l 

In the course of my nearly twenty-years studies of ore deposits I have yet met 
with no deposits (carrying sulphides) which answer to Werner's definition that is, 
whose ores are contemporaneous with the country rock and which form a regular 
interstratifled bed between other rock strata. 

Like Posepny", Pumpelly recognizes the importance of deposits which 
do not fill pre-existing cavities, devoting to these his subdivisions I and II. 
These he says fall under two heads, as regards the manner in which the 
space occupied by them was obtained : (1) by mechanical displacement of 
the inclosing material ; (2) by a chemical replacement similar to that to 
which pseudomorphs owe their origin. His use of the form as a basis of 
subdivision for deposits tilling pre-existing cavities seems more legitimate 
than in the case of those which his title seems to imply are merely concen- 
trations of metallic minerals already existing in the rock, and the use of 
the word "concentration," as applied exclusively to the latter classes, seems 
unfortunate, as implying that the others are not concentrations also. 

Geikie's classification has the merit of conciseness and his principal 
divisions are based on genetic principles, but his subdivisions, like those of 
von Cotta, recognize only differences of outward form. 

In view of the difficulty, or even, in many cases, the apparent impossi- 
bility, of determining definitely the genesis of a given deposit, it may well 
be questioned how far it is advisable to adopt genetic relations as the basis 
of a classification, since it will frequently happen that an observer will be 
at a loss to determine under which subdivision the deposit he is studying 
should be placed. It seems to the writer, however, that in such a case, 
although his determination may not be final and may give rise to discussion 
and difference of opinion on the part of other observers in the same field, 

'Op. cit., p. 423. 


he will be led by this very fact to make a more thorough and searching 
examination than if he were only required to define the deposit in question 
according to its outward form. 

As regards the applicability of the foregoing classifications to the Lead- 
ville deposits, it will be seen from a perusal of the following pages that no 
one of the subdivisions proposed would adequately define them ; either 
they would apply only to a limited portion of the deposits or else they 
would include them under the same head with deposits of an essentially 
different character. 

Of von Cotta's, Lottner's, and Whitney's subdivisions, several would be 
applicable; thus, a large part of the deposits are contact deposits; other 
parts, however, not being at the contact of two different rocks, would be 
stocks when large and pockets, chambers, etc., when small. The same remark 
would apply to Pumpelly's subdivision of his Class II, 2. On the other 
hand his definition of gash veins, as filling open fissures, would not apply 
to those of this region. The deposits would come under only a single head 
of Grimm's, Geikie's, and von Groddeck's classifications. By the two for- 
mer the^y would be classed under the general head of stocks, which really 
defines nothing except that they are of irregular shape and large. Finally, 
von Groddeck's term "metamorphic," or "metasomatic," applies to all the 
Leadville deposits and defines one most essential characteristic ; without 
some modification, however, it would apply equally well to a large por- 
tion of the Rocky Mountain deposits in Archean rocks, which have been pre- 
viously considered to be " true fissure veins." 


Manner of occurrence. By far the most important of the ores of Leadville 
and vicinity, both in quantity and in quality, occur in the blue-gray dolo- 
mitic limestone of the Lower Carboniferous formation, hence known as the 
Blue or ore-bearing Limestone, and at or near its contact with the over- 
lying sheet of porphyry, which is generally the White or Leadville Por- 
phyry. They thus constitute a sort of contact sheet, whose upper surface, 
being formed by the base of the porphyry sheet, is comparatively regular 
and well defined, while the lower surface is ill-defined and irregular, there 
being a gradual transition from ore into unaltered limestone, the former 


extending to varying depths from the surface, and even occupying at times 
the entire thickness of the Blue Limestone formation. This may be re- 
garded as the typical form of the Leadville deposits; there are, however, 
variations from it, and also in the character of the inclosing rock, which do 
not necessarily involve any difference in origin or mode of formation. As 
variations in form, the ore sometimes occurs in irregularly-shaped bodies, 
or in transverse sheets not always directly connected with the upper or 
contact surface of the ore-bearing bed or rock; it also occurs at or near the 
contact of sheets of Gray or other porphyries with the Blue Limestone, and 
less frequently in sedimentary beds, both calcareous and silicious, and in 
porphyry bodies, sometimes on or near contact surfaces, sometimes along 
joint or fault planes. 

Composition. The prevailing and by far the most important ore, from 
an economical point of view, is argentiferous galena, with its secondary 
products, cerussite or carbonate of lead and cerargyrite or chloride of silver. 

Lead is also found as anglesite or sulphate, as pyromorphite or chloro- 
phosphate, and occasionally as oxide in the form of litharge or more rarely 
of rninium. 

Silver frequently occurs as chloro-bromide, less frequently as chloro- 
iodide, and very rarely in the native state. Chemical investigation has 
failed to detect sufficient regularity in the proportions of chlorine, bromine, 
and iodine, combined with the silver, to justify the determination of distinct 
mineral species. 

A frequent alteration product of mixed pyrite and galena, which occurs 
in considerable quantity, associated with the ore bodies, is generally called 
"basic ferric sulphate." It is an ocherous-looking substance of somewhat 
uniform outward appearance, but of varying composition, being mainly a 
mixture of jarosite, or yellow vitriol, and hydrated basic ferric sulphate, 
with more or less anglesite and pyromorphite. 

Gold occurs in the native state, generally in extremely small flakes or 
leaflets. It is also said to have been found in the filiform state in galena. 

As accessory minerals are: 

Zinc blende and silicate of zinc or calamine. 

Arsenic, probably as sulphide, and as arseniate of iron. 


Antimony, probably as sulphide. 

Molybdenum, in the form of molybdate of lead or wulfenite. 

Copper, as carbonate or silicate. 

Bismuth, as sulphide and its secondary product, a sulpho-carbonate. 

Vanadium, as dechenite or the vanadate of lead and zinc. 

Tin, indium, and cadmium have been detected in furnace products. 

Iron occurs as an ore, though in the Leadville deposits in general it con- 
stitutes an essential part of the gangue or matrix in which the valuable ore 
is found. In the former case it occurs in considerable bodies as pyrite or 
sulphide and as anhydrous oxide or red hematite, with a little magnetite. 

Gangue. The other components of the ore deposits, which may be con- 
sidered as gangue, although this term is perhaps more strictly applicable to 
non-metallic minerals, are: 

Silica, either as chert or as a granular cavernous quartz, and chemic- 
ally or mechanically combined with hydrous oxides of iron and manganese. 

A great variety of clays or hydrous silicates of alumina, generally 
very impure and charged with oxide of iron and manganese, the~extreme 
of purity being white normal kaolin, containing at times sulphuric acid in 
appreciable amount. 

Sulphate of baryta or heavy spar. 

Carbonate of iron, pyrite, and sulphate of lime are comparatively rare 
in the deposits of Leadville itself. 

The miner's term, Chinese talc, has been retained for a substance 
which is found with singular persistence along the main ore channel, or at 
the dividing plane between White Porphyry and underlying limestone or 
vein material, and also at times within the body of the deposit. It is com- 
posed of silicate and a varying amount of sulphate of alumina, to which 
no definite composition can be assigned. It is compact, semi-translucent, 
generally white, and so soft as to be easily cut by the finger-nail. It is 
very hygroscopic; hardens and becomes opaque on exposure to the air. 

Distribution. With regard to the distribxttion of the above ores the prin- 
cipal generalizations to be made are: 

I. That the main mass of argentiferous lead ores is found incalcareo-magnesianbeds. 
II. That ores containing gold and copper are more frequently found in silicious beds, 
in porphyries, or in crystalline rocks. 


These associations have already been remarked in other mining dis- 

Secondary alteration. Here, as elsewhere, the ores found near the surface 
are mostly oxidized or chloridized ores, and those farther removed from it, 
or comparatively unexposed to the direct action of surface waters, are 
mostly sulphides. It may be observed, moreover, that the zone of sec- 
ondary deposition, or that in which oxidized ores predominate over sul- 
phides, varies in the depth to which it extends with the relative altitude of 
the deposit; or that in higher altitudes, where surface waters are imprisoned 
by frost during a larger portion of the year, the proportion of secondary 
products is less. 

There is a contrast in this respect, however, between the deposits of 
Leadville and those of the more arid regions of the Great Basin. In the 
latter the surface zone, or zone of oxidation, is generally more sharply 
defined and extends down to what is known as the water level. This con- 
trast is more apparent than real, for the zone of oxidation is there dry, be- 
cause of the limited atmospheric precipitation, and in Leadville generally 
wet, partly because of the relatively great precipitation and partly because 
of the peculiar geological position of the deposits, which renders them more 
accessible to surface waters. The alteration of the ore deposits is produced, 
not by the water alone, but by the atmospheric agents which it brings from 
the surface with it; whereas in the case of deposits below the water level 
the water which reaches them, not coming directly from the surface, but 
through a relatively long underground passage, has during that passage 
been deprived of these active agents of oxidation or neutralized. 

Mode of formation. From the present investigation it has been assumed, 
with regard to the mode of formation of these deposits: 

I. That they were deposited from aqueous solutions. 

II. That they were originally deposited mainly in the form of sulphides. 
III. That the process of deposition teas a metasomatic interchange with the material 
of the rock in ichich they icere deposited. That is, that the material of which they were 
composed was not a deposit in a pre-existing cavity iu the rock, but that the solu- 
tions which carried them gradually dissolved out the original rock material and left 
the ore or vein material in its place. 

IV. That the mineral solutions or ore currents concentrated along natural icater 
channels and followed by preference the bedding planes at a certain neohf/ical horizon, but 
that the;/ also penetrated the adjoining rocks through croxx joint* and clcaragu planex. 


Age of deposits. As regards the time of deposition of the original ore 
deposits, it is proved: 

That they were deposited not Inter than the Cretaceous period. 

That they are later than the inclosing rock is proved by their mode 
of occurrence; and since they have partaken of the dynamic movements to 
which these rocks were subjected, and were folded and faulted with them, 
they must have been formed earlier than these dynamic movements, which, 
as the geological considerations already presented show, occurred not later 
than the close of the Cretaceous period. 

Origin of the metallic contents. With regard to the immediate source from 
which the minerals forming these deposits were derived, the following con- 
clusions have been arrived at: 

I. That they came from above. 
II. That they were derived mainly from the neighboring eruptive rocks. 

By these statements it is not intended to deny the possibility that the 
material may originally have come from great depths, nor to maintain that 
they were necessarily derived entirely from eruptive rocks at present im- 
mediately in contact with the deposits. 

The facts and reasons on which these conclusions are based will be 
given in the following chapters. 




General description. Of the three pi'incipal groups of mines, that of Iron 
Hill presents the simplest type, both in geological structure and in the 
character of its ore deposits. It is that of a block of easterly-dipping beds, 
with a fault on its western side, by whose displacement these beds have 
been lifted in places about one thousand feet above their western continu- 
ation, and in which the ore deposition has taken place at the upper surface 
of the limestone bed, along its contact with the overlying porphyry, and 
extending down at times into the mass of the limestone. This simple type 
obtains only on the south end of Iron Hill, and even then in a somewhat 
modified form, the north presenting, as will be seen later, the extreme of 

The area represented on the Iron Hill and North Iron Hill maps forms 
topographically one continuous ridge. The map has been printed on two 
sheets, partly because of its cumbersome size and partly because 
the geological character of the opposite ends of the hill is very different. 

The Iron Hill map includes all of Iron Hill except its northern por- 
tion, together with a part of Dome or Rock Hill, the spur which lies be- 
tween California and Iowa gulches. It thus takes in all the mines belonging 
to the Iron Silver Mining Company, to the La Plata Mining and Smelting 
Company, and to the Silver Cord Combination, which represent the prin- 
cipal developments outside the Adelaide-Argentine group in this portion 
of the Leadville region. 

Iron Hill and its companion, Carbonate Hill, are flat-topped bosses or 
shoulders, on the main spur of the Mosquito Range between California and 
Evans gulches, whose form was evidently due originally to the displace- 



ment of Iron and Carbonate faults, though much modified by later erosion. 
The region, however, as distinguished from the other portions of Leadville, 
has been scarcely affected by glacial action, California gulch, in which 
erosion has been deepest, being, as has already been shown, essentially a 
valley of erosion. The slopes of the hills are steep, but extremely regular, 
and covered with an accumulation of "Slide," whose average depth may be 
considered to be from six to ten feet. This slide is distinguished from 
Wash by being not rounded, but angular and resulting from the disinte- 
gration of rock in place. It consists mainly of the debris of White Por- 
phyry, which forms the top rock of either hill. The porphyry weathers 
into thin sherd-like fragments, which from their relative lightness are easily 
carried down by rain or snow, and therefore cover the greater part of the 
slopes of the hills, even where other rocks actually crop out. It is only 
along the steep slopes of the V-shaped valley of California gulch that actual 
outcrops of rock in place are found on either hill. 

Geological structure. The average strike of the formations on Iron Hill is 
a little west of north, and the beds dip east at an angle of about 12 to 
25, shallowing, however, to the eastward, and probably basining up toward 
the Mike fault. The south face of the hill has, by the erosion of the deep 
V-shaped valley of California gulch, been left so steep that its surface is but 
thinly covered by detrital material, and east of the Iron fault, whose line is 
marked by a slight depression down the slope, the outcrops of the succeeding 
sedimentary beds can be readily traced, in the numerous prospect holes, from 
the Lower Quartzite, immediately overlying the Archean, up to the main 
body of White Porphyry, which forms the summit of the hill. 

The geological section represented on this slope is, then, in descending 
order : 

1. Wbite Porphyry capping, iii which are included detached portions of the 
Weber Shales, represented in the lines shaft by black shales and, along the outcrops 
on the Liuie and Bull's Eye claims, by a greenish slate containing plentiful easts of 
lAngula mytitoides. 


2. Blue Limestone 200 

3. Parting Quartzite (outcrop obscure) 20 

4. White or Silurian Limestone 140 

5. Lower or Cambrian Quartzite 160 

6. Archean gneiss (not exposed.) . , 


Later intrusive sheets. Besides this normal series of beds, are two intrusive 
sheets of porphyry of later eruption than the White, and allied to, though 
not absolutely identical with, the Gray Porphyry. One of these is found 
at the top of the Blue Limestone, the other near its base. Their probable 
extent can be best seen by reference to the map and sections (Atlas Sheets 
XXIII, XXIV, XXV). The thicknesses there given are assumed from the 
position of outcrops, where they could be determined, and from other in- 
direct evidence, and may differ considerably from the actual facts, as these 
porphyry sheets, especially the later ones, vary much in thickness in rela- 
tively short distances. 

upper sheet. The rock of the former of these bodies is of a dark-gray 
color and consists of plates of altered mica and relatively large, opaque, 
white feldspars in a greenish-gray matrix. So far as seen it is in a too 
advanced state of decomposition to allow of a satisfactory determination of 
its original constituents. Externally, however, it resembles more closely 
the country rock of the Printer Boy mine than any other porphyry col- 

This sheet, while in general separating the White Porphyry from the 
Blue Limestone, does not always keep exactly the same horizon. In the 
bed of California Gulch, where the outcrops cross and where this porphyry 
seems to be thickest, it cuts into the Blue Limestone, leaving a portion of 
the latter above it, near the mouth of the La Plata tunnel. Farther west, 
on the hill slopes, it cuts up into the White Porphyry for a short distance, 
leaving a sheet of that rock between it and the Blue Limestone, and then 
again returns to the contact on the Lime claim, on Iron Hill, and west of the 
Dome fault, on Dome Hill. There is direct evidence that the sheet thins or 
wedges out from this crossing of California Gulch to the south, west, and 
northj but on the east no workings have yet reached a sufficient depth to 
cut it. It is not impossible that it may be an offshoot from some large body 
occupying a lower position in that direction the Printer Boy body, for 
instance, which is at a lower geological horizon, though actually brought to 
a higher elevation by faulting. 

Lower sheet. The rock of the second body, as compared with that just 
described or with the normal Gray Porphyry, has in the hand specimen a 


much finer grain, and its minute feldspar crystals are generally of a flesh 
color. When thoroughly bleached by decomposition it can be distin- 
guished from the White Porphyry by its speckled or mottled appearance, 
whence the name of "mottled porphyry" that is not infrequently applied 
to it. It is probably also a variety of Gray Porphyry, though, like the pre- 
ceding, not found in sufficiently fresh condition for exact determination. 

As nearly as can be determined from the various prospect holes on the 
slope of the hill, this body has its maximum thickness near the line of the 
Iron fault and thins out to the southeast. It is best seen in a tunnel driven 
in near the fault, on its contact with an underlying limestone, which is sup- 
posed to be the lower portion of the Blue Limestone, though, as the Part- 
ing Quartzite was not actually exposed below it, this cannot be regarded as 
beyond a doubt. A certain amount of iron-stained material is found at the 
contact, and it had been supposed by some that this repetition of a contact 
of porphyry and underlying limestone below the regular outcrop was evi- 
dence of another fault, the different character of the two porphyries having 
escaped observation. 

This porphyry sheet is probably of much wider extent than the one 
previously described, although its actual outcrop is much more limited ; as 
will be seen later, it probably extends under the greater part of Carbonate 
Hill, and inasmuch as sheets of Gray Porphyry are found in considerable 
development on the north end of Iron Hill, though at somewhat lower hori- 
zon, it is fair to assume, as has been done in the sections (Atlas Sheet XXIV), 
that it extends under Iron Hill also, gradually lowering in horizon toward 
the north. It is probable that the small bodies of Gray Porphyry found 
crossing the limestone in various points of the hill are offshoots from this 

white Porphyry. The White Porphyry, which forms the summit of the 
hill, is the normal rock already described. From the quarry in California 
gulch, just above Graham gulch, was taken the specimen chosen for complete 
analysis (see Appendix B, Table I). In this quarry, which is but a short 
distance west of the Iron fault, the jointing planes are strongly marked, 
those parallel with the plane of the fault being the most prominent. 


Blue Limestone. The Blue Limestone, as shown by the map, has an 
unusually broad outcrop in California gulch, owing to erosion and to the 
low angle at which it stands. From the bed of the gulch the outcrops extend 
up along the hill slopes on either side, only obscured by slide or surface 
debris, until cut off by the Iron and Dome faults, respectively. On the 
Montgomery claim, a cliff exposure of a very considerable thickness of the 
lower beds is afforded by an open cut, where the limestone was formerly 
quarried as a flux for the smelters. There is also a small outcrop west of 
the Emmet fault, near the bed of the gulch, below the Columbia tunnel. 
From the upper beds in the Silver Wave ground were taken the specimens 
illustrated in Plate VI (p. 64) and whose composition is shown in Appen- 
dix B, Table V. The characteristic ribbed structure is here very well devel- 
oped. The thickness of the formation, as calculated from these outcrops, 
is two hundred feet or more, which is greater than that deduced from meas- 
urements on Carbonate Hill. 

Silurian. The White Limestone is disclosed in numerous prospect holes, 
and some shafts on the south side of the gulch have cut the character- 
istic Red-cast beds. The Parting Quartzite could not be unmistakably 
recognized, owing to its close resemblance underground to decomposed 
porphyry. There is, however, no reason to assume that it is wanting. 

Cambrian. The Lower Quartzite is best shown in the Globe and Garden 
City shafts, each of which has cut through it into the underlying Archean. 
The quartzite is of the usual normal type and the Archean is a coarse- 
grained granitoid gneiss. 

iron fault. The average direction of the line of the Iron fault is a little 
east of north, but its course is very crooked, as shown on the map. Although 
this irregularity may be somewhat increased by erosion, i. e , be greater than 
if the line given on the map were its intersection with a horizontal plane, 
still it cannot be considered abnormal, since from the bed of California 
gulch northward to the Codfish Balls shaft it has been actually proved in 
so many cases as to render its delineation unusually exact. 

It has been* cut by the workings of the Garden City shaft; by the L. 
M. shaft, which was sunk perpendicularly to the depth of two to three 


hundred feet through White Porphyry, on the west side of the fault, into 
Lower Quartzite on the east side; by two shafts on the Lingula claim ; and 
by numerous shafts and winzes in the claims of the Iron mine, some of the 
latter being sunk on the plane of the fault itself, and showing its average 
dip to be 60 to 65 to the westward, or nearly at right angles to the dip 
of the formation. 

As the Blue Limestone has not yet been reached on the west side of the 
fault in the region represented on this map, its movement of displacement, 
or throw, cannot be accurately determined. Its maximum is probably not 
far from one thousand feet, since the Cit} of Paris shaft, 1,200 feet north 
of the line of the map, was sunk to a depth of 800 feet without reaching 
the Blue Limestone. The dip of this bed carried back from the outcrop 
on Carbonate Hill, at the average angle, would reach at the line of the 
fault a much greater depth, probably not less than fifteen hundred feet ; 
but there are good grounds for assuming that this dip shallows, and that 
the beds actually basin up, i. e., assume a westerly dip, before reaching 
the line of the fault. The movement of this fault may here be partly dis- 
tributed among smaller parallel faults to the west, like the Carbonate fault, 
in which case the contact immediately adjoining the main fault may be found 
at a less depth than 1,000 feet. To the north, beyond the limits of this map, 
as has already been soen in the general description of the Leadville region, 
the movement of the Iron fault gradually decreases and it apparently passes 
into an anticlinal fold. As regards the continuation of the fault south of 
California gulch, however, no definite data have been obtained, since the 
great accumulation of Wash and Lake beds there have been a barrier 
to underground explorations. It has been assumed that it gradually 
passes into a synclinal fold, as indicated on the map of Leadville. The 
movement of displacement south of California gulch is, however, distributed 
among two faults, the Dome and the Emmet, with which the Iron fault is 
connected by a cross-fault (the Californki fault), which follows approx- 
imately the bed of California gulch. 

California fault. The plane of this fault has not been actually cut, but its 
existence is proved by the discrepancy of the beds on either side of the 



gulch, the Blue Limestone outcropping near the Robert Emmet tunnel and 
opposite the Globe shaft, in which the Lower Quartzite is cut. 1 

Dome fault. The Dome fault is in one sense the proper continuation of 
the Iron fault, since it forms the great break on Dome Hill, as Iron fault 
does on Iron Hill, and, like the latter, passes at its extremity into an anti- 
clinal fold. Considered in this way, the Iron, California, and Dome faults 
would form a single fracture, somewhat irregular in direction, but having a 
general north-and,-south trend, while the southern continuation of the Iron 
fault, as at present indicated, and the Emmet fault, would be simply 
branches, relieving the strain at the sudden bend of the fault in California 
gulch. To the east of this line of fracture are the principal outcrops of Blue 
Limestone and the main ore developments in this region, while to the west 
this horizon is more or less deeply buried beneath a covering of porphyry. 
The Dome fault proper has a general nort'i-and-south direction. Its plane 
has been proved by underground workings only in the Vining tunnel, but 
the line as given on the map is tolerably closely determined by the develop- 
ments of adjoining shafts and inclines, those on the west finding White 
Porphyry, underlaid by Gray Porphyry, on a level with Blue Limestone on 
the east, in the Rock and Dome workings. 

Emmet fault. This small fault, running in a southwest direction from 
the California fault, has a movement of displacement the reverse of the 
majority of the faults in this region that is, the upthrow is to the west in- 
stead of to the east. Its plane has actually been proved by a drift running west- 
ward from a winze sunk in the Robert Emmet tunnel. It is further proved 
by the discrepancy in the position of the Blue Limestone and the overlying 
porphyries on either side of it, as shown in Section G, Atlas Sheet XXV. 
That it actually continues to its junction with the Iron fault to the south, 
as indicated on the Leadville map, is merely a matter of conjecture. 

Dome Mm. By reference to Atlas Sheet XXV, Sections E and F, it will 
be seen that the northern portion of the ridge of Dome Hill, adjoining Cal- 

1 Since the close of field-work, developments iu tho Garden City mine have definitely located the posi- 
tion of the western end of this fault. The lower shaft on this claim was sunk perpendicularly 100 feet 
through limestone aiid vein material, and then passed into the Lower Quartzite, crossing the fault 
diagonally. At 120 feet a drift to the southwest cut the fault at 5 feet from the shaft, showing that 
its dip is to the south. At 75 feet from the *haft the same drift cut the plane of the Iron fault and passed 
into i lie White Porphyry on the west side of this fault. 


ifornia gulch, was originally pai't of the Iron and Carbonate Hill ridge and 
that their present separation by the valley of California gulch is due tc 
erosion since the Glacial epoch. What is now the main crest of the ridge 
was once an arm or bay in the Arkansas lake, and the actual rock surface 
is buried to a great depth beneath the deposits formed in this lake and the 
later Wash Except, therefore, on the northern edge of the ridge adjoining 
California gulch, which is the portion shown on the Iron Hill map, data 
with regard to the actual rock surface are extremely meager. Its geological 
structure above and to the east is similar to, and practically a continuation 
of, that of Iron Hill, namely, a series of easterly-dipping beds, capped by 
porphyry, in which the ore bodies have been developed by following the 
contact of the Blue Limestone with the overlying porphyry. The main 
difference lies in the development of the intrusive sheet of Gray Porphyry 
below the White Porphyry, which is not, however, absolutely parallel 
with the bedding, inasmuch as on the summit of Dome Hill a small sheet 
of White Porphyry is left between the Gray Porphyry and the limestone 
and in the La Plata ground the Gray Porphyry cuts down through the 
upper part of the Blue Limestone. 

West of the Dome fault the relative position of these two sheets of 
porphyry affords most valuable evidence as to the underground structure, 
and actually proves a basining-up of the beds towards the Dome fault, as 
has been assumed to be the case in regard to the beds west of the Iron 
fault. At the Bank of France shaft the Gray Porphyry actually comes to 
the rock surface. The City Bank and Oro City, on the other hand, pass 
through the White Porphyry into the Gray, as does the Vining shaft higher 
up on the hill. The Sullivan, Ben Burb, and Keno shafts have reached 
the contact and limestone after passing through the White and then a com- 
paratively thin body of Gray Porphyry. The Blue Limestone is thus 
shown to be at no great depth below the surface near the Dome fault. On 
the other hand, at the Coon Valley shaft, near the head of Georgia gulch, 
the Blue Limestone is over six hundred feet deep, showing a comparatively 
steep dip from the fault westward. 1 

'Since the completion of field-work the contact and even valuable bodies of ore have been proved 
in this region west of the Dome fault, notably in the Rosie, Sequin, and Vining claims. In the Sequin 
the contact was struck at 375 feet, in the Viniug at 317 feet, in each case with a sharp dip to thewest- 


The wedge-shaped block of ground between the Emmet and Iron 
faults may be considered a portion of the formation which, by compression 
between the adjoining blocks, has been lifted up relatively and compressed 
into an anticlinal fold. Actual outcrops of Blue Limestone are found near 
the bed of California gulch, opposite the Globe shaft. The Columbia tunnel 
was run in apparently on the very crest of the fold and developed consid- 
erable ore on the contact. From the line of the tunnel the formation dips 
gently to the eastward and very steeply to the westward, so that in the 
Crescentia shaft,, a little west of it on the slopes of California gulch, at a 
depth of 335 feet the limestone had not yet been reached, but the shaft was 
in the Gray Porphyry beneath the White. 1 Section G, Atlas Sheet XXV, 
represents graphically the structure thus described. 

Ore deposits. The principal deposition of ore has taken place along the 
contact-plane between the Blue Limestone and overlying White Porphyry, 
and extended to greater or less depth into the mass of the limestone. In 
several instances large deposits have been formed within the body of the 
limestone, being probably on the line of some natural cleavage or joint 
plane which caused a deviation of the ore currents from their normal 

The vein material or gangue consists of hydrated oxides of iron and 
manganese, silica, and clay. The iron varies from a hard, compact, more or 
less silicious brown hematite to a simple coloring matter of the clay. Man- 
ganese is found sometimes in fine, needle-like crystals of pyrolusite, but 
mainly occurs as a sort of wad, a black clayey mass known to the miners 
as " black iron." Silica occurs either as a blue-black chert or as a granular, 
somewhat porous mass, hardly distinguishable from quartzite. Clay is 
found in greatly varying degrees of impurity, from a white kaolin down, 
and is a product of the decomposition of porphyry. It occurs either in 
place or as an infiltrated mass. Besides this should be mentioned the 
Chinese talc of the miners, found mainly at the actual contact. 

The ore is principally argentiferous galena and its secondary products 
are carbonate of lead, or cerussite, and chloride of silver. As accessory 

1 Late developments in the lower Garden City shaft show that the Blue Limestone is considerably 
mineralized and that the formation dips very steeply to the southwest. 


minerals, or those of less frequent occurrence, are sulphate of lead or 
anglesite, pyromorphite, minium, zinc blende, and calamine. Native sul- 
phur is found in one instance as the result of the decomposition of galena, 
and native silver formed by the reduction of chloride. 


The principal mine workings in the area represented on the Iron Hill 
map may be divided into the following groups, commencing at the south: 

1. The Rock and Dome. 

2. The La Plata and Stone. 

3. The Liuie and Smuggler. 

4. The Silver Wave and Silver Cord, including the South Bull's Eye. 

5. The Iron mine proper, including the North Bull's Eye. 

Rock and Dome. These two claims are owned and worked by the Iron 
Silver Mining Company. The former is opened by a tunnel running south- 
ward on the strike, the latter by an incline running eastward on the dip. 
The ore bodies thus far developed in either mine are found near the sur- 
face of the hill and may belong to the same bonanza, if the same north- 
easterly direction of ore shoots prevails here as does on Iron Hfll. On the 
hillside, at the present mouth of the Rock tunnel, was formerly an actual 
rock outcrop, consisting largely of hard carbonate, from which the mine 
derived its name and where the first ore in place was found in this region. 
From it were no doubt derived the heavy fragments which caused so 
much annoyance to the early gulch miners. 

From this tunnel level the ore has been followed along the contact of 
limestone and porphyry a certain distance upward or toward the outcrop, 
but mainly eastward in the trough of a fold and then downward on the dip 
The workings have also been pushed southward with the intention of mak- 
ing a connection with the Dome workings. Beyond the crest of the fold 
to the eastward the contact has thus far proved comparatively barren, but 
at the lower extremity of the Rock incline ore has been found which may 
be the precursor of a second ore shoot. 

In the Dome the rich ore has thus far been found near the mouth of 
the incline, in very considerable thickness and with a remarkable develop- 


ment of masses of Chinese talc in the ore body, at some distance from the 
contact. The incline has not yet reached a second ore shoot in depth, 
though there is every probability that one will eventually be found there. 

The ore in both these mines is mainly a hard carbonate, very rich in 
lead, but of comparatively low grade in silver. It is very thoroughly oxi- 
dized, and in some cases a red oxide of lead has been found in it. It 
occurs in bodies sometimes of considerable thickness and always at or near 
the contact. At the contact the alteration of porphyry into the so-called 
Chinese talc is very persistent, and when found in the ore body, as in the 
Dome mine, shows that offshoots of the porphyry had probably penetrated 
the limestone previous to the replacement of the latter by vein material. 

Sections E and G, Atlas Sheet XXV, which pass through the Rock 
workings, show the fold in the limestone, which affords a good illustration 
of the tendency of the ore currents to deposit their load immediately above 
any sharp bend in the stratification. 

La Plata, stone, and A. Y. The La Plata claim is opened by a tunnel 
800 feet long, running south from near the bed of the gulch. Its direction 
was intended no doubt to correspond witli the strike of the formation, but 
in point of fact it diverges a little to the westward, so that while at the 
mouth it is at the actual contact of the White Porphyry and Blue Limestone, 
it departs from it more and more as it advances. At the extremity, however, 
the contact bends sharply down to the south, so that a winze has been sunk 
70 feet to reach it. It is noticeable that this bend is on aline with the east- 
ward continuation of the California fault. 

Below the mouth of the tunnel and in the body of the limestone is 
found the Gray Porphyry sheet, which to the north and south is found 
above the Blue Limestone and separating it from the White Porphyry. The 
contact in this mine was not found veiy productive. A small body of ore was 
found east of the tunnel, near its mouth, and a prospecting drift running to 
the Gneisson shaft, and continued some distance beyond it, found the usual 
evidence of mineralizing action, but no pay ore : it showed, however, a 
steepening of the dip of the formation of 35. This, with the sudden steepen- 
ing at the end of the tunnel, shows how difficult it is to count on any regu- 
larity in the dip of the formation until it has been actually proved. The 


main ore developments have been in the body of the limestone, extending 
as much as one hundred feet below its surface, and are opened by the Rus- 
tin shaft. These and the similar ones in the Silver Wave ground are inter- 
esting as showing that the ore deposits